In minetest 5.1.0, players onboard a train are not located in the right
position. Currently on the latest version of advtrains, they are located
too high. I am unsure whether the fix will require an advtrains change or
a minetest engine change but if it is in minetest engine then this bug
should at least be left open until the relevant engine fix.
This is solved in the branch mt5-fixes.