How do I know which size joiners to get?
You can buy joiners by rail code (height of rail in one-thousandths of an inch.) Some joiners fit more than one code. And a number of codes are used in more than one scale.
Jim
The rail joiners that Atlas offers are made for both code 83 and code 100 rail. If you like Peco track, then you really should follow previous advice and get joiners based on track code. I believe Micro Engineering joiners are like Peco, you must buy based on rail code.