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Great Western 0-4-0 tank engine R.077 or R.333 (1978 to 1983), motor is a Hornby X.908 (Johnson 111), it has contacts on both wheels via springy metal that brushes the inner wheels.
The coupling rods should be parallel to the rails. They needn't be at a quarter turn offset between left and right sides, this is only needed for realism (steam engines would have the pistons offset to ensure one side always had 'drive'.
The top can be removed without unscrewing anything.
To disassemble fully, remove top, remove weight, remove motor-retention spring clip, remove motor, remove motor mount after unscrewing from the bottom.
The wheels are pressed onto the axles, and can be prised off carefully. When reassembling, check the free-flow of the wheels when driven from the front axle. There should be no sticking points as the wheels rotate, if there are then adjust the wheel rotation so that the coupling rod is parallel to the rails.
If the wheel spins on its axle, then epoxy or glue may be needed to hold it in place, though it may be better to just replace the wheels and/or axles.
For old motors, it's can help to clean the rotor shaft with a small spray of contact cleaner and apply a tiny amount of oil.
This was bought from more or less a rummage bin for a few quid, it actually runs quite nicely after being overhauled. I had assumed a motor winding was burned out because it was stalling a lot, especially at low speed, but after re-aligning the wheel coupling rods, cleaning the motor and gearings, and ensuring clearance between wheels and chassis, it started running well a all speeds.