Is it legal to have a paintball gun on board a train?

Answer 1

Transporting the markers is fairly straight forward, they need to be in your checked luggage and sometimes (not always) they will have you declare them as firearms for transportation, make sure whoever is checking in your bags knows they are in there.

The real problem is actually any tanks you are bringing with. To transport them you have to remove the regulator/pin valve from the top so when they are checking them before you fly they can see inside of them. If you leave them on, do not expect your tank to be in your baggage when you land.

Sometimes it is better to ship your gear ahead if it is an option

Answer 2

It is legal, as long as your tanks are not filled, and your marker, paint and tanks are in separate parts in your bag, just like the laws for cars.