Hi. It looks to me that his tyres are slipping. When you showed the under of Thomas, his tyres looked really old. First thing I would do is replace his rubber tyres.
Hope that helps.
At first thought I had agreed with AOF but you said in the video's description that you changed the tires. The tires in the video look like they don't sit proud enough from the plastic of the wheel where it wouldn't have enough grip on the slope. Did you use a new rubber or just ones from another train? My second thought was that in addition to the tires, the slope needed a good washing to restore the traction but I think you said this particular engine slips on all slopes. The engine looks very strong and probably not any gear slippage, looks like its entirely within those wheels. I would be curious if swapping out the axles and wheels of one of the other engines would narrow down the investigation.
is the UK one heavier than the others? it's probably an issue with traction because it's not really stopping.
dreaming of piles of track
I would say give him new traction tyres since those look like they've hardened but you said you did that already. Obviously not a gear problem since his wheels keep moving, very strange...
TrackMaster Expert (kinda)
I'm pretty sure I know what the problem is, I have the same problem with my Back n go Thomas, on wich I also replaced the traction tyers. The problem is the traction tyers. I can seet that you replaced the plarail ones with Trackmaster ones and there is a difference between them. When plarail tyres get too worn they break down, harden and get stuck to the wheel, but this doesn't happen for Trackmaster ones, instead the harden and lose all their elasticity, so the no longer grip the wheels, so Thomas may actually run better without them. You can test if this is the case, by blocking the wheels from turning and seeing if you can spin the tyre around with ease.
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