EASY PEASY for a change
Malc - we have the answer to your poser (I think):
1. Train out of sidings, pull carriage B down and push into sidings.
2. Train then goes anti-clockwise round track, and pushes carriage A to position B.
3. Move train back to bottom of track and pull carriage B out of sidings and leave on the bottom of the track. Train continues round track clockwise pushes carriage A up next to carriage B.
4. Send the train round the track in the other direction (anti-clockwise), as it comes to the bottom then push both carriages into the sidings - A will go into the sidings first, B second.
5. Pull carriage B out of the sidings and round to position A. Train carries on round the track. Pull carriage A out of the sidings and push round to position B.
6. The train can then go back down from that position and into the sidings!
(That was from Stan the Ram

)
Lillian