Perhaps leaving the gold at the right hand side of the freight car and walking over to the door. How can we solve this problem without doing much more work?
So the trick is to regard the gold as part of the freight
car. Note that we didn't have to assume a symmetrical mass distribution
for the freight car. So this should work fine. Eqn. 1.44
should be changed a litte. 
is no longer the combined
mass of robber plus gold, since the robber is now separated
from the gold. This term should be . The denominator
is altered two ways. is reduced by the mass
of the gold, but the mass of the freight car 
is increased
by the same amount. So the denominator is unaltered.
We get then
So he'll make it if he abandons the gold. Of course he could still take 100 kg of gold with him and get out alive, but we're all honest citizens and won't tell him that. His new career as a physicist will be enough of a reward.