Do you consider fair evasion theft?

Not a great analogy given fixed space on a bus or a train; in rush hours a fare evader occupies a seat that would otherwise be occupied by a a fare payer (presumably).

Media piracy is largely associated with either people that were never going to pay for a cinema seat or DVD, OR (and this is key) people that would likely pay for something were it available ...

I assume you mean "fare" evasion -- like riding a train or bus?

Yes, it fits. There are a finite number of seats on the train, and your gratis use of a seat (ostensibly) denies another paying customer. Even if there's excess capacity at the time of your ridership, the train operator is designing their capacity with buffer, so you're essentially stealing the capacity.

You could just as easily peg the theft to the incremental cost (electricity, gasoline, etc) it takes the train operator requires to move your incremental mass from A to B.

This is distinctly different from infinite, free copying & distribution.