What are the restrictions on picking up locomotives?
It varies by which Ticket To Ride game you're playing.

In USA, Europe, and Asia, you may do any of the following on your turn:


Draw a face-up locomotive
draw two face-up non-locomotive card
draw two cards off the deck
draw one face-up non-locomotive and one card off the deck, in either order.


In Switzerland and Nordic Countries: 


draw two face-up cards
draw two cards off the deck
draw one face-up card and one card off the deck, in either order.


The difference has a lot to do with how they are used - the Switzerland and Nordic rules don't allow engines to be used for plain routes, only for tunnels and ferries.