The entrance into Platte canyon, in the foothills, is private property, gated and signed. You'll first find the grade by driving up from Morrison (as I recall), and you can backtrack a short ways downstream on the grade.
As you continue west past Bailey, you'll see the grade in several places. The rails and wye at the summit are worth stopping.
You can also see (but not drive upon) most of the downgrade west, right up to Como. You'll want to have a r.r. map to see everything. The Boreas pass grade is usually good (in summer), with the rotary plow and number 9(?) at the bottom just before you get to Breckinridge.
The other route, heading southwest from Como, can be seen for a few miles. A little bit can be driven, but basically you'll be takng roads that occasionally cross the grade, or bring yourself close enough to say "yeah, that's probably the grade".
You might also want to buy the video made by the local South Park folks, which features the last passenger train from Bailey to Breckenridge. Sorry, don't have the title handy.