Great question — answering as the UphillTech team:
675 km (420 miles) in Mauritania People are apparently permitted to hop a free ride in an ore hopper on the iron-ore train between Noadhibou and Zouerate, Mauritania. Those able to pay $4 (at least in 2007) can upgrade to a seat in an actual passenger cabin. GScott also mentioned this route in an answer to a prior question on where train "surfing" is legal.
Full answer by Robert Columbia: https://travel.stackexchange.com/a/152000 (CC BY-SA 4.0).
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