Getting over a footbridge with my dodgy knee is bad enough, especially with both hands full. I'm currently in my twenties, so heaven knows what that'll be like after forty more years of wear and tear.
This whole issue comes up on a regular basis. I don't know if there was ever a situation where the friendly porter would ask the signalman if anything was due, and then happily wheel the chair-bound passenger over the barrow crossing to the other side. However, I remember one occasion when a chair-bound passenger from Weymouth was trying to get out of Castle Cary, and the original suggestion was 'go to Westbury, then come back'. However, any train from Westbury would have landed him right back on the island platform. Going to Taunton was apparently the only option, until someone remembered that the barrow crossing existed.