Inquiring minds, who may one day do this innocently, want to know.
It seems so:
Also
There is a difference between a crime and an offense, shirley …
So, if you find yourself in a car with a group of unseemly characters and you fear you may become a victim, it’s illegal for you to move to another car?
Is this law enforced as vigorously as the one against jaywalking? 
*I must’ve done this a thousand times in my pre-2014 youth.
Every subway car has a group of unseemly characters.
The likelihood of getting ticketed for moving between cars is very low. People do it all the time.
If it’s prohibited, why is it possible? I mean, without tampering with the locks, triggering some emergency switch or kicking out a window?
Huh? / not snark.
And my name is Leo.
Hasn’t changed since 1971, when Sylvester Stallone made Woody Allen nervous: (short clip): https://youtu.be/blOUfHFaNvs
The gangway doors (with the exception of 75-foot-long cars) are unlocked so that passengers can travel to the end of the train in case of an emergency evacuation.
On 75-foot cars they are locked because the large end-excess of the cars on curves would make traveling between cars extremely dangerous. Those doors can be unlocked remotely if there is an emergency.
The upcoming R-211 order will at long last have open gangways.
I think what Bones Daley meant was that there is a difference between prohibited acts that are punishable by jail or prison time and prohibited acts that are punishable by a fine. The OP asked if walking between subway cars was a “crime,” a word that most people associate with a criminal offense that is punishable by imprisonment. But this act is only punishable by a fine, not imprisonment, and that’s likely the distinction that Bones Daley was pointing out (i.e., this act is not a crime in that sense of the word).
I myself, or, I, for one, have never thought that “crime” was connected to only one type of punishment, which, as we know can take many forms for many fits.
Other people besides Bones really walk around with that definition? Am I in a minority?
Not worth a new OP.
The word is civil infraction. Like a parking ticket. It is not a crime, it doesn’t go through the criminal justice system, you don’t have a right to a jury trial, but it’s illegal, and enforced by the gubbermint. A distinction without a difference most of the time, unless you’re arguing a point of law.
Plus a civil/administrative infraction by itself doesn’t lead to you being arrested and booked, but merely issued a citation, so it doesn’t give you a criminal/arrest record.
“What do they want for their lousy 35 cents. Live forever?”-Walter Mathau- The Taking of Pelham 12 3 
I just looked at this list of possible offenses.
“Carrying liquid in open containers: (fine of) $25.”
Seriously? How many times have I, and millions of others, ridden in the NYC subway system carrying in a backpack or handbag, an open plastic bottle of water?
a related question:
I don’t care whether it’s legal or not…I just want to know WHY people walk between the cars?
I’ve only been to NY a few times in my life, but whenever I took the subway, there was constant activity, as (mostly young) people opened the doors and walked into and out of the car I was sitting in.
What are they looking for?
In most cases, my car was not overly filled up.
And not once did I feel any problem of safety or perceived threat from other passengers.
I really couldn’t find any logic to it.
Maybe some native subway denizens could tell me something about the local culture?
I can’t speak for all train-car-wanderers, but here’s my take based upon my experience as a subway traveler. Many subway stations have exits to the street located at opposite ends of the platform. As well as exits in the middle of a platform which are more appropriate for changing trains.
So occasionally you might want to line up your subway car with the exit point you prefer. People who make the same trip every day will know to stand at a certain point on the platform to be in the front, middle or end of the train depending on their exit point. Occasionally for one reason or other folks miss, and they then walk through the train to reach the best exit position. Might save them hassle of doing it above-ground in the streets or on a crowded platform.
Just my take on one possibility.
On the London Underground, it is not a crime, but very risky, as there is open space between the carriages. That said, the new stock on the “Sub Surface Lines” (Circle, District, Hammersmith & City, Metropolitan) is all open, walk-through and Air conditioned. When the “Deep Tube” lines eventually get new stock (the Bakerloo has stock from 1972, the Piccadilly from 1975) it will almost certainly be open as well. It’ll be a while before any further lines get their stock replaced; generally London Underground trains tend to be in service for 30-50 years.
The new Elizabeth line will be walk-through, air conditioned and driverless. The platforms will have doors that only open when the train is present.