MTH makes a nice set of military passengers (30-11065). I don’t see any on the FIND IT LOCALLY tab, but that doesn’t mean they aren’t out there. These are nice, but at $3 per figure, it would probably cost more than the cars to populate the 6 cars with these.
If you don’t mind painting them, you could get the 120 figure set (30-11043). About half would be WACs. Another option is to get some china ebay figures and paint them.
I’ve seen another set of painted army figures that include a few benches, but can’t recall who made them…maybe RMT, but there are none om their site at this time.