Inter-city buses are the cheapest way to travel if you don't own a car. For example a train ticket NYC->Washington DC is over $100. A bus ticket for the same trip is $30.
The train will be more comfortable and possibly faster, though.
Not aware of anything comparable cost-wise, especially bringing a fully assembled ready to ride mtb along.
Annoyingly however, even a Greyhound bus does the trip faster than the Zephyr. Such is the sad state of passenger rail in the US.
Source: I've taken that train at least 150 times at this point.
If you want a sightseeing tour, that's another matter, it's great for that.
Amtrak is ok for a leisurely vacation stroll across the country; it's not an alternative for regular business travel.