Questions, is there a possibility that there is an "insider" on the train? That is, that someone gets in without a ticket? How long would it take to be discovered?
And how would this work for, so to speak, ghosts? Do they also charge souls?
What about pets? Can things like dogs or bittys get on the train?
1. not unless they were like, invisible. The train is very secure and you will be checked for ticket or pass very thoroughly. Never say never though, maybe someone has some magic powers to fake a ticket or somthn. Unless you’re otherworldly in hiding you’d probably be figured out quite quickly and depending in who finds you the outcomes will differ, policy is to kick off train though.
2. Tickets are sold per body! So people like Fresh, Frisk sharing a body with Chara and a monster that absorbed all the human souls from their au would only need to buy one ticket. Ghosts like Napstablook are the exception to this rule, as they'd need to buy a ticket as well.
Tickets are generally paid for in G, with the exception of one-way passes, which are free of charge. Those are more of a "pay us back if you want to, but there's no obligation" situation.
The only reason G is charged to get onto the train is maintenance and supply costs. The train cannot keep running if it is broken, after all.
3. yeah animals are allowed but may be placed in a specific part of the train because of potential allergies. Bitties would probably go as a passenger as they’re sentient usually.