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CN Software Engineer Interview Guide

CN Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

As a Software Engineer at CN, you are not just writing code; you are building the digital infrastructure that keeps a critical North American supply chain moving. This role sits at the intersection of complex industrial operations and modern software development, requiring you to bridge the gap between traditional engineering domains and scalable technical solutions. You will contribute to projects that enhance operational efficiency, safety, and reliability across our vast rail network. The work you do impacts thousands of daily movements, from optimizing logistics to maintaining the health of our massive equipment fleet. You might find yourself working on simulation tools, system architecture, or data-driven applications that provide real-time insights to our operations teams.

This guide is scoped to a Software Engineer candidate at CN.

Focus areas

Root Cause Analysis (RCA)
FMECA (Failure Modes, Effects, and Criticality Analysis)
RAM (Reliability, Availability, Maintainability) Domain Know

The CN interview process

CN candidates report 4 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Initial Screening

Begins with a pre-recorded video assessment or a recruiter phone call.

What to demonstrate

Begins with a pre-recorded video assessment or a recruiter phone call
Depth in Root Cause Analysis (RCA)

How to prepare

Be able to walk your CV end to end in two minutes, and say why this company specifically.
Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Technical Conversations

Deeper technical discussions with hiring managers and technical peers.

What to demonstrate

Deeper technical discussions with hiring managers and technical peers
Depth in Root Cause Analysis (RCA)

How to prepare

Answer aloud and timed: Have you ever changed the minds of your colleagues after they were initially opposed to an idea you presented?
Answer aloud and timed: What is one mistake you made at a previous job, and how did you fix it?

Behavioral Conversations

Engagement in standard behavioral inquiries related to team dynamics.

What to demonstrate

Engagement in standard behavioral inquiries related to team dynamics
Depth in Root Cause Analysis (RCA)

How to prepare

Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
Re-read the description of the behavioral conversations above and write down what you would ask to confirm before it.

Collaborative Interviews

Interviews involving multiple team members or stakeholders from different disciplines.

What to demonstrate

Interviews involving multiple team members or stakeholders from different disciplines
Depth in Root Cause Analysis (RCA)

How to prepare

Answer aloud and timed: What is the difference between a struct and a class?
Answer aloud and timed: How do you conduct an RCA (Root Cause Analysis) and FMECA (Failure Mode, Effects, and Criticality Analysis)?

Where candidates lose points

Prepare your stories

Use the STAR method (Situation, Task, Action, Result) to structure your behavioral answers. Keep them concise and focused on your personal contribution.

Know your resume

Every project listed on your resume is fair game. Be prepared to explain the "how" and "why" behind every technical decision you made in those projects.

Be ready for technical breadth

Even if you are applying for a specialized role, be prepared to answer questions about general engineering concepts like Linux, containers, or encryption.

Research the domain

Showing an interest in how software impacts rail operations or industrial logistics will set you apart from candidates who only focus on the code.

Questions & practice

Collapse a redelivered event batch into per-aggregate high-water marks

Easy · hashing · at-least-once · aggregation

You drain a batch of up to 5,000,000 events, each (aggregate_id BIGINT, aggregate_version INT, event_type, payload). The log guarantees order within one aggregate only; the batch merges 64 partitions, and a relay failover has redelivered a range, so an older version for an aggregate can appear after a newer one. Given a map of last_applied_version per aggregate, produce the events worth applying, at most one per (aggregate_id, version), plus the count discarded. Target O(n) time. State the memory for 2,000,000 distinct aggregates and what you do when it does not fit.

Approach

One pass, one hash map from aggregate_id to the highest version kept, and a discard counter. An event whose version is at or below last_applied_version for its aggregate is dropped without further work, which is the whole reason the event carries its version rather than a delta. O(n) expected time, O(d) space in distinct aggregates.
Keep the maximum, never the last occurrence. The redelivered range means the final appearance of an aggregate in the batch can be an older version than one seen earlier in the same batch, so last-wins applies stale state over newer state and the projection regresses with no error anywhere.
Cost the memory instead of calling it large: an 8-byte key plus a 4-byte version is 12 bytes of payload, and an open-addressed table held at a 0.7 load factor costs roughly 17 bytes per entry before per-slot metadata, so 2,000,000 aggregates is tens of megabytes in a native layout and several times that in a runtime that boxes both key and value.
If the distinct set exceeds memory, partition on hash(aggregate_id) mod P and reduce each partition independently. Every event for one aggregate hashes to the same partition, so the per-partition result is exact and the merge is concatenation rather than a second reduction.

Follow-up

The payload is a patch rather than a snapshot, so applying only the highest version loses the intermediate changes. What changes in your reduction?
How do you detect that version 7 arrived while version 6 was never delivered, and what should the consumer do about the gap?

Identify the heaviest tenants in a five-minute window under memory pressure

Medium · top-k · heavy hitters · streaming

The edge service handles about 3,000 requests per second across roughly 50,000 tenants, peaking near 9,000. Expose the 50 heaviest tenants by request count over the trailing five minutes so limits can be tightened before one tenant's backfill starves the fleet. You may not retain five minutes of raw records. Give the exact solution and its memory, then the bounded-memory approximation with its error stated as a formula, and say which you would ship and at what tenant cardinality that choice changes.

Approach

Do the exact version first, because it is affordable at this cardinality: a ring of 300 one-second counters per tenant, advanced lazily, is 1,200 bytes of counters per tenant and roughly 60 to 90 MB for 50,000 tenants with overhead. Carry a running total and subtract the bucket you overwrite so a window read is O(1) rather than 300 adds.
Extract the top 50 with a size-k min-heap over the tenant sums: O(d log k) for d tenants, against O(d log d) to sort them all. Maintaining the heap continuously instead of on query requires a tenant-to-heap-index map, because incrementing a count already inside the heap means sifting from a known position, and without that map you rebuild the heap on every request.
State the approximation precisely rather than gesturing at sketches. Misra-Gries with m counters retains every item whose true count exceeds N/(m+1), and each retained count underestimates the truth by at most N/(m+1). With m = 1,000 and N = 900,000 requests in the window the error is roughly 900 requests, which is fine for spotting a tenant sending 50,000 and useless for ranking two tenants 200 apart.
Say what breaks when the window slides: Misra-Gries and Space-Saving are insert-only and cannot be decremented as records age out. The workable construction is one summary per sub-window, say ten seconds, with 30 summaries merged at query time, and the merged error is the sum of the per-summary errors, so the bound degrades linearly in the number of sub-windows.

Follow-up

The heaviest tenant is heavy because of one export job rather than user traffic. Should the limiter treat those as the same tenant?
Two tenants sit tied at the boundary of the top 50. Does your answer flap, and does the flapping matter?

Find overlapping job attempts and peak concurrency from lease records

Medium · sweep line · intervals · leases

A day of job_run history yields about 50,000,000 attempt records: (job_run_id, job_type, attempt, started_at, finished_at which is NULL when the worker died, lease_expires_at). Leases expire on a clock, so a job that outran its lease ran twice. Produce (a) every job_run_id whose attempts overlapped in wall-clock time and (b) the peak number of simultaneously running attempts per job_type with the minute it occurred. Target O(n log n). State how you treat a NULL finished_at and what clock skew does to your answer.

Approach

Define the interval before sorting anything: an attempt occupies [started_at, COALESCE(finished_at, lease_expires_at)). finished_at is observed and lease_expires_at is only a promise, so every attempt without a finish contributes an estimate and the whole result is a lower bound on overlap rather than an exact count.
For peak concurrency, sweep: emit 2n endpoints, sort by (timestamp, kind) with ends ordered before starts at equal timestamps, then walk the sequence maintaining a counter per job_type and record each type's maximum with its timestamp. O(n log n) dominated by the sort, O(n) space, or O(1) extra if the sort is external and the walk streams.
For overlap detection, do not compare attempts pairwise. A single global sort by (job_run_id, started_at) gives both the grouping and the order; within a group, keep the maximum end seen so far and report an overlap exactly when the next start is less than that running maximum, which is one linear pass after the sort.
Half-open intervals matter and are easy to get wrong: with closed intervals an attempt ending at the same millisecond another begins reads as concurrency two, and across 50,000,000 records that artefact swamps the real signal.

Follow-up

A handler is not idempotent and you have found 400 overlapping jobs. Which of them actually caused damage, and what would you query to find out?
Peak concurrency for one job_type is 4 against a configured cap of 4. Is the cap working, or is the data hiding attempts that never started?

Write the update path that detects a concurrent edit

Medium · optimistic concurrency · transactional outbox · audit history

resource carries version INT NOT NULL DEFAULT 1. resource_revision holds revision_id, resource_id, version, actor_user_id, change_kind, patch JSONB, request_id, created_at with UNIQUE (resource_id, version). outbox_event holds aggregate_type, aggregate_id, aggregate_version, event_type, payload, status. A PUT carries the version the client read. Write the exact statements for the single transaction that applies the edit, records the revision and enqueues 'resource.updated', and give the handler's branch on zero affected rows. Then say what PostgreSQL 16 does under READ COMMITTED when two of these updates hit one row at once.

Approach

One transaction, three writes, no network call inside it: UPDATE resource SET title = $3, version = version + 1, updated_at = now() WHERE resource_id = $1 AND tenant_id = $4 AND version = $2; then INSERT the resource_revision row at version $2 + 1; then INSERT the outbox_event row at the same aggregate_version. The event goes to a table rather than a broker because no transaction spans both.
Branch on the affected-row count before doing anything else. Zero has three causes — stale version, wrong tenant, row gone — so re-read once and map to 409 carrying the current version, or 404 for an id outside the caller's tenant, which also stops the endpoint confirming that another tenant's id exists.
State the engine behaviour instead of assuming it. Under READ COMMITTED the second UPDATE blocks on the row lock, and when the first commits PostgreSQL re-evaluates the WHERE clause against the newly committed row, so the version predicate now fails and the statement reports zero rows. Under REPEATABLE READ the identical collision raises SQLSTATE 40001 instead, so the handler must fold both shapes into one conflict response.
Keep UNIQUE (resource_id, version) even though the predicate already serialises writers. It is what makes a lost update unwritable if any other path ever reaches the revision table, and it converts a logic bug into 23505 rather than into a silently missing history row.

Follow-up

A client sends the version it read ten minutes ago and the resource has moved three versions. What is in your 409 so it can resolve the conflict without a full re-fetch?
Two editors, two disjoint fields, no overlap. Does your answer still refuse the second write, and should it?

Replace offset paging on the resource feed with keyset

Easy · keyset pagination · composite index · sort order

resource holds resource_id, tenant_id, owner_user_id, title, body_ref, version, status ('draft','active','archived','deleted'), created_at, updated_at, deleted_at, with an index on (tenant_id, status, updated_at DESC, resource_id DESC). The listing endpoint returns active resources for one tenant, newest update first, 50 per page, today with LIMIT 50 OFFSET n. Tenants reach page 400 and rows are created while they read. Write the keyset query, define what the cursor carries and how it is encoded, and say which part of the index each predicate uses. Assume PostgreSQL 16.

Approach

Name the two failures separately. OFFSET 20000 makes the server produce and discard 20,000 rows, so page cost grows with depth rather than with page size. Independently, any write that changes how many rows sort above the offset moves the window between two fetches, and the direction decides which anomaly you get: an insert lands at the head of updated_at DESC and pushes already-returned rows down past the boundary, so they are returned a second time; a delete above the offset, or a row whose updated_at is bumped above the cursor, pulls rows up and one is never returned at all. Nothing in the response reveals either.
Write the seek: WHERE tenant_id = $1 AND status = 'active' AND (updated_at, resource_id) < ($2, $3) ORDER BY updated_at DESC, resource_id DESC LIMIT 50. The row-value comparison is one index range rather than a disjunction, and both columns are NOT NULL, which is what makes that comparison well defined.
Map each predicate onto the index: tenant_id and status are equality on the leading columns, (updated_at, resource_id) is the range, and the ORDER BY matches the index order so no Sort node appears and the scan stops after 50 rows. The DESC in the definition only matters for mixed directions — a plain ascending btree on the same columns is read backwards for this query.
Put both sort columns in the cursor and nothing the client can tamper with into another tenant: base64 of (updated_at, resource_id), validated server-side, with tenant_id taken from the principal.

Follow-up

The client asks for 'jump to page 400'. What do you offer instead, and what does the honest version cost?
Sort order becomes user-selectable across four columns. How many indexes is that, and which would you refuse to add?

Explain the difference between Java and C++.

Medium · Technical Proficiency and Domain Knowled

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
Work from the requirement backwards to the design.

Follow-up

What assumption would you test first?
How would you know your answer was wrong?

What is the difference between a struct and a class?

Medium · Technical Proficiency and Domain Knowled

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
Work from the requirement backwards to the design.

Follow-up

What assumption would you test first?
How would you know your answer was wrong?

How familiar are you with technologies like Ansible, Linux, containers, or encryption?

Medium · Technical Proficiency and Domain Knowled

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
Work from the requirement backwards to the design.

Follow-up

What assumption would you test first?
How would you know your answer was wrong?

Explain a project on your resume in detail, including the tech stack and API design.

Medium · Experience and Problem-Solving

Approach

Say who the caller is and what they do when the call fails halfway.
Define the identity of a request so a retry cannot double-apply it.
Separate accepted, pending, failed and confirmed; they are different facts.
Design the error taxonomy before the success shape; callers branch on it.

Follow-up

What happens if the caller retries after a timeout?
How does a client discover it is on an old version of this contract?

If you were not familiar with a specific language or framework, how would you go about learning it?

Medium · Experience and Problem-Solving

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
Work from the requirement backwards to the design.

Follow-up

What assumption would you test first?
How would you know your answer was wrong?

If you had to refactor code left by a developer who is no longer at the company, what steps would you take?

Medium · Experience and Problem-Solving

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
Work from the requirement backwards to the design.

Follow-up

What assumption would you test first?
How would you know your answer was wrong?

What app or website annoys you the most, and how would you improve it?

Medium · Experience and Problem-Solving

Approach

Clarify what is being asked and what a complete answer contains.
State your assumptions explicitly before working the problem.
Say what you would check first and why it is the highest-information step.
Work from the requirement backwards to the design.

Follow-up

What assumption would you test first?
How would you know your answer was wrong?

How do you conduct an RCA (Root Cause Analysis) and FMECA (Failure Mode, Effects, and Criticality Analysis)?

Medium · Technical Proficiency and Domain Knowled

Approach

Establish what changed and when, before forming any theory.
Pick a bisection that eliminates candidates whichever way it turns out.
Check the instrumentation before believing the symptom.
Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

What would you look at first, and what would it rule out?
How would you tell a cause from a coincidence here?

What tools would you use to troubleshoot a network-related issue?

Medium · Technical Proficiency and Domain Knowled

Approach

Establish what changed and when, before forming any theory.
Pick a bisection that eliminates candidates whichever way it turns out.
Check the instrumentation before believing the symptom.
Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

What would you look at first, and what would it rule out?
How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics CN candidates report.

Day 1 — Map the CN loop

Write out the reported sequence: Initial Screening, Technical Conversations, Behavioral Conversations, Collaborative Interviews.
For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 4 reported rounds, with the weakest marked.

Day 2 — Work Root Cause Analysis (RCA)

Spend the session on Root Cause Analysis (RCA), which CN candidates report being tested on.
Write one worked example in Root Cause Analysis (RCA) and time yourself on it.

Deliverable: One timed worked example in Root Cause Analysis (RCA).

Day 3 — Work FMECA (Failure Modes, Effects, and Criticality Analysis)

Spend the session on FMECA (Failure Modes, Effects, and Criticality Analysis), which CN candidates report being tested on.
Write one worked example in FMECA (Failure Modes, Effects, and Criticality Analysis) and time yourself on it.

Deliverable: One timed worked example in FMECA (Failure Modes, Effects, and Criticality Analysis).

Day 4 — Work RAM (Reliability, Availability, Maintainability) Domain Knowledge

Spend the session on RAM (Reliability, Availability, Maintainability) Domain Knowledge, which CN candidates report being tested on.
Write one worked example in RAM (Reliability, Availability, Maintainability) Domain Knowledge and time yourself on it.

Deliverable: One timed worked example in RAM (Reliability, Availability, Maintainability) Domain Knowledge.

Day 5 — Answer out loud: Behavioral and Cultural Fit

Answer aloud, timed: Why do you want to work for CN?
Answer aloud, timed: Tell me about a time you had to handle a difficult employee or coworker.

Deliverable: Spoken answers to 2 reported Behavioral and Cultural Fit question(s), under time.

Day 6 — Answer out loud: Technical Proficiency and Domain Knowledge

Answer aloud, timed: Explain the difference between Java and C++.
Answer aloud, timed: What is the difference between a struct and a class?

Deliverable: Spoken answers to 2 reported Technical Proficiency and Domain Knowledge question(s), under time.

Day 7 — Answer out loud: Experience and Problem-Solving

Answer aloud, timed: Explain a project on your resume in detail, including the tech stack and API design.
Answer aloud, timed: If you were not familiar with a specific language or framework, how would you go about learning it?

Deliverable: Spoken answers to 2 reported Experience and Problem-Solving question(s), under time.