Riverside, CA to Denver, CO buses and trains from $118

Here are some trips other travelers have found on Routefriend.
$118
20h 10m
1 stop
Greyhound
$172
1d 0h
1 stop
Amtrak
$125
21h 17m
2 stops
Amtrak Greyhound
$178
1d 1h
2 stops
Greyhound Amtrak
$197
1d 7h
5 stops
Greyhound Amtrak