March, will be weather dependent, for the south rim of the Grand Canyon; North rim will probably still have snow; Death Valley should be ok, cooler there, in March , than in May & June, late may in DV, I've hit temps of 117f.
March travel in the area of Rushmore and Yellowstone. Bring a snowmobile.
When you say the Rockies By train, are you departing from Vancouver B.C.? Is that your drop point for the second trip?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.