3.5 GB of memory usage by TS2017 does appear to GENERALLY be a threshold, but I have found with particular routes/scenario combinations, a failing situation can be resolved with a fresh reboot of the PC - it clears out some of that dross that builds up in the background over time
I also find that manually closing down some other un-needed background applications helps (e.g. Dropbox, OneDrive, mailwasher (my email filter), and Chrome will often leave random traces).
When the memory is "on the edge" I always keep the Windows Task Manager window open on my left monitor, on the Processes tab, click the Memory (Private Working Set) column to put highest usage at the top, and keep an eye on the RailWorks.exe number - when it gets over around 3.2 GB, it's an indication that things are getting tight, and if nothing else an F2 SAVE is called for
