> And using the RER which is dirty, smells like piss and full of graffitis, overcrowded at peak hours and often on strike is so unreliable that it is hardly an enticing option.

I was surprised that the RER doesn’t have bathrooms on-board. In Ontario Canada, we have similar double-decker commuter trains and they have them. They’re about airline-style in terms of size (barring 1 “accessible” one per train), but they do the job.