The FAQ you're reading on their HART website in regards to baggage on the TRAIN is misleading because the baggage size restrictions and rules about fitting onto your lap or under the seat aren't stated. 

The Bus currently DOES NOT allow any baggage that can NOT FIT on your lap or under a seat so even a carry-on suitcase in most cases would NOT be permitted. Here is the EXACT policy from the bus:
http://www.thebus.org/howtoride/RulesReg.asp?l=
Also this as well, but this one is less detailed:  http://www.thebus.org/howtoride/BaggageRules.pdf