Update on the steam thing. I realised that, with the valve gear the machine had, reversing would be a bit of a problem. Also, I felt the drive gears did not reduce the revs of the crankshafts sufficiently for the machine to have much torque. Therefore, a major redesign of the valve system and power transfer gears has taken place, as per the still below.
The reversing system lacks a link rod to the (as yet unmodelled) lever in the control cabin, and the control cabin itself remains almost entirely unmodelled. I have scripted the motion of the rotating things, including a decelerate/direction change/accelerate routine, but the motion of the valve and piston links will have to be keyframed. Subsequently I doubt this will be ready before the deadline, in which case the earlier YouTube link will stand as the entry. I do plan to finish this project, however, irrespective of the contest deadline.