Indeed you should be able to use the same sensors as you already use for your obstacle detection: add two that point sideways, and one of them should always see the wall. If no more wall, stop, as you're on the platform. The gap detectors should take care of that part, guiding you towards the platform part (as there's a gap on both sides), so when you don't detect a wall on either side it means you're on the platform. You may need to move a bit further forward of course to get off the track, but you then know exactly how much.
Yet another option: don't bother with detecting the wall. The moment you detect gaps all around you (resulting in a very narrow track), you can tell you're on the platform. Unless there are any parts of the track that have no obstacles and are no wider than this platform bit.