I've heard the migration only works if you have sufficient space on the new drive, and no picking and choosing what gets copied. In this case, my existing HDD is 1TB (with about 600 GB used), whereas the new SSD is 275GB (kind of a strange number for computer stuff). I wouldn't be able to do that without deleting a substantial amount of stuff. It would be nice if I could just copy the OS by itself.
Matt
Before you do anything, I highly recommend getting a backup drive too like one of those Desktop jobs from Seagate. They're inexpensive and come with gobs of Terabytes of space.
Don't bother to use their backup program and instead make a folder called Backups!!!, or something like that so you know not to delete it. I did that with a folder called DO NOT DELETE!!! and in there I have stuff like my downloads I want to keep, backed up documents, and other data. The folder is obvious so I don't do the inevitable stupid mouse click that's associated with a brain fart!
Anyway. Copy the stuff off like pictures, downloads, documents, music, etc. I'll bet this is where most of your drive space is anyway. T:ANE too is probably a big hunk of this data. Remember to grab that App Data/local/N3V Games folder too as that's your data.
Once that's done, and you have ensured your data you want to keep, including your Trainz stuff, delete the data. Gulp! Yup get rid of it since you have the backup. If you're not sure measure twice, or three times then make the cut.
Once you've done that, you'll find that you'll have plenty of room to fit the OS plus programs on to the SSD. My current 240GB, I know what you mean about drive size, has 72GB free.
It sounds a lot worse than it really is, but this is the only way around it unless you want to do a fresh install on the SSD of your OS plus other programs. That is unless of course there are other computer icky issues you need to clean up with a fresh install. A fresh Windows 10 install is pretty easy to do unlike the other OSs we've used, but all in all I can't reiterate enough how important it is to have a backup of everything before you start. As that will ensure you've got yourself covered. If I lived closer I'd visit and gladly help out.