Hello, please can you tell me how I would book to ensure the disabled wheelchair bound person can travel on the RER A from Paris to Disneyland on 01/09/2017. Thanks Tara
RER trains don't have assigned seats so there aren't special seats for wheelchair travel. It is recommended to board at the head of the train though, as the conductor has a special ramp he will use to bridge the platform gap for the wheelchair, allowing ease of roll-on / roll-off.
I've added a wheelchair accessibility guide to the article on reduced mobility access on Paris trains. It gives a visual walkthrough of how to take the RER when using a wheelchair. It's in French but there are photos for each step which should help give you an idea of what the trip will entail.
There are wide fare gates and elevators accessing the train platforms at all the central Paris RER A stations nowadays, although you will have to find the entrances (there are many, but not all of) which have elevators. The accessibility guide gives you an example of what to look for at station entrances to know it's wheelchair accessible.
I hope this helps!