Why is 'dans le train' correct but 'dans le RER' is not (it is 'sur le RER')?

Question

Accepted Answer

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" ;-)
Je suis sur les lignes du RER = I am on the Tube [US: Subway] lines 
I hope this is helpful.
Bonne journée !

Answer

The “RER” (Réseau Express Régional) is the full railway and transit system, not specifically the vehicle of transport.  You are  ‘in the train’ - dans le train, but not ‘in the system’. Ultimately though, as with all things grammar and prepositions, the reason is ‘because that’s the way it is’.