Could you make the deceleration factor a mod setting so we can change it to whatever we want?

Good idea. Currently its nearly immediate stop. And in the original mod its realisitic deceleration. A setting would be something in between.

The difference between my mod and the original mod is the fast decelleration.

If you prefer a realistic decellaration you can use this mod: https://mods.factorio.com/mod/TrainSpeedLimit_018_Port

I introduced the change since I found it annoying that the train needs ages to decellerate. You can use multiple signs with decreasing speed limits to get s smoother slow down as well (200, 150, 100 instead of only 100 for example).