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 !
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’.
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