Seat classes

[Baldy's Boss]

How difficult would it be to have passenger carriages that had different fare classes?...or to make the age of a passenger train car influence the fare paid by those who travel on it?

[Supercheese]

You could modify the cargo_age_period property of different coaches to change the effective value of cargo transported in them over a given time interval.

While not exactly 1st class/2nd class, it does offer a distinction between coaches.

[Sylf]

Perhaps you can write a town newgrf that generates two types of passengers? Replace mail with first class passengers, but still code them as passenger class, so the train sets won't use the mail graphics.

[Emperor Jake]

Tourists in ECS already sort of do this, they don't have as many destinations as your average commuter though.