Hello!
If you follow /r/citiesskylines on reddit you may know about a small application coming out lately, called NetworX. It's an app allowing you to make public transport diagrams! It's still an early versions but as far as i know it will be updated (it already was at least once today, and the programme came out out yesterday). When you install it it comes with an example map of Munich (München). I find it very helpful, it's definetely better than drawing stuff in MS Paint ;) . It is free so check it out! Link to the software's webpage and it's subreddit is down below.
Greetings and have fun!