Hello!
You can set any resolution by editing the „setting.lua“ which can be found as described under point 11. here:
http://www.train-fever.com/forums/topic/most-common-issues/
Open it with a text editor and just change the windowSize values to read like this:
windowSize = { 2569, 1080 }
Please make a backup copy in advance or if anything goes wrong, you can delete the file and it should be restored by the Steam client.
Alternatively you can use a tool like TGMM where you can set resolution and TGMM will edit the settings file for you:
https://www.train-fever.net/filebase/index.php/Entry/486-Train-Fever-Game-Manager/
Please let me know if this worked for you!
Best regards
Tom