The 90% bit is what makes it difficult.
Basically, there are these options:
1) A terrain grid with 50 m step width. You can download it as raster file at 50 m resolution or as vector data showing the contour lines.
In the first case, you end up with a lot of .asc files. These can be opened in any editor. The data format is dead simple. The header says where the tile goes and how big it is and then you have the data points, in their text representation. Top-left to bottom right, with top being north, of course.
Contour lines are in GML format which is also easy to process but you don't need that for our purpose anyway.
https://www.ordnancesurvey.co.uk/openda ... tml#TERR502) LIDAR data at 1 m resolution. It comes as DTM which stands for Terrain and DSM which stands for surface. The first is what you want. It comes in the same .asc format but you select a tile first on the website and download just that. Again, you find a set of .asc files in the ZIP.
https://data.gov.uk/dataset/lidar-composite-dtm-1m1Both options share one problem: They use the (or some?) national grid and not UTM. That means that translating that to in-game coordinates is homework to be done. I have not looked into this deep enough. What you find on the web looks quite mathematical and I am not into installing tons of software packs just to find out whether they work or not.
Ordnancesurvey.co.uk also have OS Open Map - Local which comes as vector data in GSM (or ESRI Shape, if you prefer that). It shows roads, present-day railway lines, shorelines etc.. Buildings are shown as simplified contours, just a rectangle for a row of houses.
This latter source gives you the option to develop a new route to original UK coordinates. You can go from the UK grid directly to game coordinates. Of course, the Google Earth overlay in route editor becomes useless then. But I might speak to fast, with some fiddling, one might find ways to set up the route origin in such a way that it fits your home grown imports.
Bottom line: Yes, there is much hot stuff out there on the net, but no, nobody has yet hacked any ready-to-use tools for earthly train simulators.