einsteigen vs. steigen:
In many cases you will be able to use either "steigen" or "einsteigen" when speaking about transport:
Sie stieg in den Zug = Sie stieg in den Zug ein.
"steigen" is mostly used as "to rise", "to climb", "to increase" - so for everything which gets up.
As you (usually - at least in old times) had to climb a couple of stairs to enter a train, we use "steigen" as well as "einsteigen", which more focuses on the fact, that the person than is "in" the train.
When the conductor gives instuction, they will always use "einsteigen": "Bitte einsteigen. Türen schließen".