@snowflitzer. I keep getting that problem whenever I add one or two more cargo train stations like in a commercial zone or something. Right away there is a huge flux of spammed trains. In a year two in-game it usually stabilises. Sometimes though once it should stabilise, I have to break the track to force them to despawn and start over. It's a bit of a cascade effect that sometimes needs to be reset.

It would be my pleasure to help out snowfitzer.

I might need your help with my trains :} my internal cargo lines are over filled with cargo trains

One thing I learned real quick when I started in 2015??? was to separate the passenger train lines completely from the cargo lines. Now I haven't even put in passenger lines though I may just for fun. Not throughout the city though, the "metro" does the same thing for the interior of the city.Though I miss all my trains from Sim City 2000, 3000, and 4. ;-)

In my main city, Roesone, I'm on the verge of improving my rail yard. I like what you've done here. I keep the to/from terminals completely cut off from road traffic. Industry can load near where they're at then transfer at the to/from Away yard. And vise versa. (Commercial too) Really keeps the trains flowing. I actually have funding at minimum (50%) because there's too much trains. So I've probably got awhile to go before needing to expand other than the to/from Away terminals.