Using the Nvidia control panel I had V-Sync turned on to force TS to have it since the screen tearing was unbearable. However very recently I've noticed in cabs my framerate would half. I have an FPS limit set to 59 (before some people chip in, it didn't solve V-sync so I had to enable it anyway to get rid of screen tearing). If you look at the framerate counter you'll see it jumps between 58 and 59 FPS, but in the below screenshot from the cab you can see it is roughly half that. I tested further. I turned off V-sync entirely and the framerate got to 51fps in the cab, which is roughly what I expected. Then I set it to fast mode, which kept the same framerate as before. Fast mode has V-sync off until the game starts producing frames beyond the refresh rate of the monitor, but I am yet to see if it helps screen tearing. Adaptive mode is something I haven't tried but probably won't based on its description. Throughout the test the framerate outside of the cab stuck at the limit, so I have no idea what is causing the drop in the cab with V-sync on. It is not a train-specific issue as it happens with all trains and routes no matter the quality. But I also can't figure out why V-sync would halve the framerate in the cab of a train but not in other views. Very weird.
That's the way V-sync works in all applications: if it can't keep stable display refresh rate (59/60 Hz), it will switch to the half rate of 30 FPS. So it means Your system can't keep stable 60 FPS while in cab. Try another way: switch V-sync off and limit max FPS to 60. Also it prevents GPU buzz in main menu as it has no FPS limitation.
This is mine. Admittedly bit of an older version, and my last days of Windows 8.1. I don't have any cab- or other specific fps drop. I do, however, get a drop when looking at certain hi-fi assets (such as the ECMLS Class 801 or the coaches supplied with the E18 pack). I certainly can have 50fps, not just 15, 20, 30 or 60. The five settings below are Off, On (which is selected), Fast, Adaptive with tearing (limiter), Adaptive to half. The rest is default, on which tab it says it's just the default, not override. I'm a bit hesitant to finally install Win10, but once I do (soon), I'll report back if it messes with functionality. In case of many old games it's reported to break them, hence staying on this veteran version (don't break what works), so who knows.