Some of the larger Amtrak station have red caps who can help you on an doff the train if you have mounds of luggage. Bags that heavy cannot go overhead - they might fall off and kill someone.

They can help you load them at the end of the car - you keep only a carry-on in the overhead rack. (I know they have them in NYC and I think DC - not sure otherwise). I highly reco you give them a decent tip - at least $10 for all the stuff you have- so you don;t have to haul it up and down escalators to the train platforms.

Can't help on planes - I have never flown with that much stuff.

There is no free baggage allowance on your internal flights except for the small carry-onboard piece.

United: 1st bag is $25, 2nd bag $35. Each additional bag is $100 and also there is a $100 per bag fee for any bag that weighs more than 23kg.

Alaska charges $20 each for your 1st 3 bags and $50 for a 4th.

If you get the United Airlines Credit card, your first bag is free on each flight as well as the first bag of one companion. This may add up to a decent savings if you have a few flights.

And remember that those extra fees are per trip, each way. For example if you fly NYC to LA with a change of plane but on one ticket, that counts as one trip. If they are separate tickets, I imagine that it would count as 2 trips.

Luggage in the US can be quite inexpensive at stores like TJ Maxx and even some department stores like Macy's. Would it make sense for you to travel with fewer bags and just buy what you need as you need it, preferably as late in your travels as possible?

It depends on the airline is the short answer to your first question, so I strongly suggest you block out some time to call and ask the airline(s) yourself. I wouldn't trust the well-intentioned answers here to be correct.

NewbE, I assume that clarkgriswold copied the information from the airlines websites and that OP Daneille08 is just trying to understand what she's already read. It does get confusing
No matter what, you need to count the number of flights, note which are on whicn airline, and count the number and size of bags.

http://www.aa.com/i18n/travelInformation/baggage/baggageAllowance.jsp

http://www.alaskaair.com/content/travel-info/policies/baggage-checked.aspx?lid=nav:travelInfo-policies
It also notes "When your travel involves multiple airlines (including flights marketed by one airline and flown by another), please refer to your travel confirmation to determine which airline's baggage policy applies"

I meant no disrespect to anyone's answers, nyer. It is indeed confusing.

I understand NewbE,and I'm one of those who advocates going to the source when you need certain information rather than relying on opinions. I think what I picked up on is that the OP had some figures but just couldn't quite see what it all adds up to in the end.