i never took real notice of those thicker lines, but thanks to neldots suggestion i just devised an experiment:
i just built a rail line to a station at a height of 490 m (so i thought, assuming that water level would be at 0 m), then i ran a train up there. first i thought the achievement would unlock immediately after opening that line, but at first it did not unlock. however, as i ran my train up that mountain, suddenly the achievement was shown to be unlocked exactly 100 height metres before i thought it should happen.
so that offers two conclusions:
1. water level is at 100 m
2. the thick contour lines indeed mark 100 m and thus the thinner lines are spaced at 10 m intervals
we can also deduct from that, according to some rough calculations regarding height lines and track segment length, that the maximum gradient in train fever is about 7 to 7,5 percent and the medium gradient thus is 3,5 to 4 percent. that translates to roughly 1:13 and 1:27 gradients for those used to uk measureing.
the highest mountain peak on the hilly map that i used to test this is 650 m by the way, so you can definately have high ernough mountains yeol.