I hope you'll find this a useful addition to the already comprehensive list:
triptocarbon.xyz is not dedicated to rail service but instead allows to compare different modes of transport using emission factors (the numbers are not specifically for DB!)
ecopassenger.com is the backend for UmweltCheck but unfortunately there isn't programmatic access at the time of writing. They are however considering adding/allowing that.
Follow up from https://github.com/dbsystel/Bahn-API/issues/4
I hope you'll find this a useful addition to the already comprehensive list:
Thank you for maintaining the repo!