OK, from my save standing in Freeside, without having discovered the Union Pacific map location, I typed 'COC Springvale' and warped successfully to the Capital Wasteland.
My map had changed to the Capital Wasteland, and I had selected my one and only active quest, 'Return to Yes Man' for the Hoover Dam Wild Card ending. The waypoint was correctly routing me to the Union Station, so getting to the train was easy. I sneaked my way there, and outside of killing a few radroaches and a mirelurk king near the train station, the only other things I did were 1) discover the locations 'Dukov's Place' and 'Union Station', and 2) get ran up on by Bryan Wilks and forced into dialogue. I went ahead and exhausted the dialogue options and received the quest 'Those!'. After receiving the quest I didn't return to my pipboy until getting back to the Mojave.
I entered the station, bought a ticket, and saved my game on a fresh file right before entering the train. I successfully arrived back in the Mojave, and opened my Pip Boy. DC was still the map on display, despite having returned. I closed it and attempted to leave the station back into Freeside, which caused the game to crash on the loading screen.
I reloaded, and this time took the train to the Mojave, then immediately purchased a ticket and rode the train back to DC successfully. Upon arriving the correct radio stations became available, and I had the text prompts for the five DLC packs pop up and the related quests became available in my Pip Boy. The map was the Capital Wasteland, however this time the Yes Man quest was no longer routing properly, and was acting as if the map were displaying the Mojave. Not an issue, obviously, so I switched it to 'Those!' which seems to be routing correctly.
Anyways, I let all the text and quests go across the screen, saved at the exit into DC, and am about to begin the game. First mission on my list is to go to Vault 101 and try to trip the main quest somewhere at the beginning, I saw a thread on here about it so I'm going to read some threads here before moving forward.
Thanks for the assistance, Jax, I'll playtest the game for a little while and as long as it's in decent working order (definitely going to get the main quest going) I'll make up a guide on how to get everything in order.