Depends on the region. Different countries have different signals and gradient limits. Where/what are you modeling?

im doing a fictional i jsut need to know the best type of gradient i can do im using donner pass rails

Well, heavy freight trains usually don't traverse anything steeper than 2%. Mountainous routes are usually between that and 1%, while the flatlands wouldn't see anything more than 0.5%. High speed raillways such as those in Britain and Germany have even less of a gradient.

Actually, that's not true. Some of the german high speed lines have gradients well above 1%. Most notably the new high speed line (maximum line speed: 300 kph) between Nürnberg and Erfurt, which contains sections of 1.7% and even 2% gradients. Another example would be the SFS Mannheim-Stuttgart (max line speed: 250-280 kph) with a maximum gradient of 1.25%.
One of the steepest gradients on high speed lines can be found on the Köln-Frankfurt high speed line (max line speed: 300kph): 4%!

And of course there is the Lickey line in the UK - about 2.6% over 2 miles or so I believe. So to the OP, the best gradient to START with when going uphill, is probably flat and gradually increasing, but probably not going much steeper than about 3% (ish) - unless you are building a roller coaster of a route.

Ya learn something new every day. :D

yeah it depends on the region to i thing the durgango and silverton has a 1.8% grade