Estimation question

Estimate how many trains operate in the NYC subway.

Practice this question out loud. An AI interviewer asks it, follows up like a real interviewer would, and scores your answer. Type or speak.

What this question tests

Tests structured estimation for a specific, real transit system using ridership and train-capacity assumptions instead of a guessed number.

State the approach: estimate via peak-hour ridership divided by train capacity and frequency, since this is more grounded than guessing train count directly.
Assume the NYC subway carries roughly 4 million rides on an average weekday, with peak-hour ridership concentrated in a few hours, say 20% of daily riders in the morning peak hour.
Assume an average train car holds about 150 people at capacity, with roughly 8-10 cars per train, giving roughly 1,300 riders per train.
Divide peak-hour riders (roughly 800,000) by capacity per train (1,300) to estimate roughly 600 trains actively running to handle peak demand at once.
Sanity-check against the known public figure that NYC's subway fleet has roughly 6,400 cars total, implying a plausible few hundred trains active simultaneously during peak service, consistent with the derived estimate.

Uses a demand-based estimation method (ridership divided by train capacity) rather than guessing the fleet size directly.
Explicitly reconciles the estimate against a known public figure (NYC subway's total car fleet), showing critical self-checking of the result.
States every assumption clearly, e.g. 'assume 150 riders per car', so each link in the reasoning is inspectable.
Distinguishes trains actively running at peak from the total owned fleet, which includes reserve and maintenance trains, a nuance many answers miss.

Guessing a train count without any derivation from ridership or capacity data.
Confusing 'trains operating at peak' with 'total trains owned by the system', which are different numbers.

How would this estimate change for off-peak hours versus rush hour?
What's the largest source of error in your estimate, and how would you tighten it?