good point about using a correct key, James. i should have added that the screwdriver would be useful to check the soundness of the mechanism, but it certainly shouldn't be taken as a long-term solution.
one thing i noticed in the video, and it might have to do with the lack of a proper key, but the spring doesn't seem to be wound very much as i cannot see it expanding as it winds down. that might be the cause of the pulling power you see lacking. held in air, the last few windings will spin freely, but on the track, the locomotive will typically slow to a stop as the spring winds down. of course you want to be careful not to overwind a clockwork motor*, but i have a feeling in your case, the spring needs to be wound quite a bit more than you have shown in the clip.
cheers...gary
* i should probably add that i've wound up Marx motors many 100's of times and have never broken a mainspring. being kid-tough was apparently a high priority in their design.