Basetiles for track are, for instance tiles 1011, 1093 and 1175 for rail, monorail and maglev respectively.
Monorail and maglev are thus 82 and 164 sprites "higher" than normal rail. Adding those numbers to 3981 gives you 4063 and 4145, which are the exact sprites shown.
Steps to reproduce
load either North Korean station set or Dutch Stations Addition Set Part 2.
Select Monorail (it's blatantly obvious there)
For the North Korean set look at the "Non-Track Tiles" tab.
For the Dutch set look at the "Non-track tiles" tab.
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Depending on the railtype the sprites may get additional offsets:
Action 1 sprite (first ground sprite)
Action 1 sprite (other sprites in the layout)
So, TTD sprites and the first ground sprite are affected by the railtype, while other action 1 sprites in the layout are not. The offset "82" refers to the offset between the default TTD track sprites; if you are using non-track ground sprites which are not from an action 1, you need to supply fake spritenumbers which preemptively reverse the offsets (that is, you need different sprite layouts for every railtype your station is available for).