I would definitely translate that as "that train". The definite article here in English is not optional, and it such cases it doesn't correspond to a demonstrative pronoun in Czech.
by placing "nastoupily" at the end, you are putting so much focus on it that it would mean "Four tourists did get on the train." accepting it here would do more harm than anything else.
Yes, in the sense that both are accepted in this exercise, since the English sentence does not indicate gender. Otherwise, turista is masculine, and turistka is feminine.