I just found a fix. In the PAINT/IMPORT window, delete the DEM\SRTM in the source field, and type it back in again DEM\SRTM. That fixed it for me....
I'm having the same problem in TS19. I have inserted the .hgt file in Program Files (x86)/Steam/Steamapps/Common/Railworks/DEM (have to create it)...
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