I think you can take an early train and get off at
Cambridge and have a day there at the
Fitzwilliam. For the Ashmolean at
Oxford, you may
still find it easier to take a very early train to
London and go to
Oxford from there. I have very dreary memories (and someone stole my suitcase) of a stopover at Bletchley trying to go from
Cambridge to
Oxford on the hypoteneuse, so to speak. But that was in 1959! Better yet, get a BritRail three-day or week ticket whenever you get a holiday, since you're young enough to use
Youth Hostels. Gosh, at your age, you can even bike and hike. Compared with the Pacific Northwest, southern
England is quite small. Maybe there are very few trains now.
Pat L.