The elastic "pipe" looks like it has been stretched. On my 1650 it is straight, not curved like yours. It looks like it represents the rod that connects the reversing mechanism to the front engine. If it is interfering with running in curves I would just remove it. Sometimes extra detail gets in the way of operation.
The front and rear engine drive rods don't have to be in any particular position. On a real articulated the front and rear engines are independent. Either can slip independently of the other so the drive rods could be in any position relative to the other engine. It makes no difference on a model either.
As far as the hopping of the front engine goes - make sure none of the brake shoes are rubbing on the drivers, especially the set with traction tires. If that all looks good you are going to have to remove the boiler top. Rotate the flywheel by hand slowly and see if you can see or feel any binding. You may have to remove the front engine to do any further checking. Remove the screw that secures the front engine to the rear engine on the underside of the engine. Then from the top remove the screw that holds the front engine to the boiler. Be careful to not lose the spring and washers and remember how they go back together. Remove the front engine so you can rotate the driveline to the front gear box by hand and look for binding.
My guess is since the "pipe" is stretched someone removed the front engine previously.
Ken
