Amtrak Cal Zephyr timetable ( [www.amtrak.com] ), ignoring of course the usual timekeeping issues:

WB lv Reno 0836 arr Roseville 1257 - 4 h 21 m
EB lv Roseville 1135 arr Reno 1606 - 4 h 31 m
Timetable miles: 134
Average speed WB: 30.8
Average speed EB: 29.7

No listed stop in Sparks any more, but figure about 10 more minutes plus Reno station dwell time.

Remember, Reno is at about 4500 ft. elevation; Roseville is at about 165 ft. (almost at sea level). So the train has much more climbing eastbound than westbound.

Presumably, Z trains would be similar, or maybe even slightly better given no stops (Amtrak stops at Colfax and Truckee). Most of the line has speed limits of 40 or less, with only a few short segments where passenger speeds are higher than freight.