KEMTRO is the best tool for planning subway trips when in Paris and London.
The interactive map helps you plan your route along any of the subway lines, and the itinerary view shows the details of the route, connections and stops. You can also see your current location to find the nearest subway stations and landmarks. A complete map is available so you can see the names and locations of all the stairs, escalators, entrances and exits for each subway station.
All this is available offline so the information is available while traveling underground and no extra roaming charges would be incurred.
See all the details of the route, at what station you need to transfer lines, and the estimates of how long your trip will take.
Select a station from the route to zoom in and see where the entrances and exits are. The detailed map and detailed exits are available after you download the free complete map file.
Major landmarks are available on the map and for route planning.
Maps and route planning available offline. No "surprise" roaming charges at the end of your trip.