Nowadays you can "negotiate" relatively easily ad hoc on a website these days. Assuming there is free capacity. On "bug" cargo routes that typically works few hours advanced.
If there is capacity, they are obliged to provide it on a non-discriminatory basis. If there isn't capacity, then it gets into questions of allocation (which still needs to be done in a fair and documented basis).
https://www.dbinfrago.com/web/schienennetz/leistungen/trasse...