Bonjour Kathryn,
When talking about a means of transport, i.e. le métro / le bus / le train, you will use 'dans'. If you are talking about the 'train lines / subway lines' etc (= les lignes), which is the case here, then you will use 'sur' because you cannot physically go inside the trainlines/ subway lines as in a carriage. That'll be a bit "mad" ;-)
I hope this is helpful.
Bonne journée !