I search this fic for a while...

I think Buffy and Spike doesn't have their power (I don't remember why), and they're in another dimension, it's like Farwest, with a train line in progress or a train, anyway I think there is a train in the story and they sleep in a wagon?(maybe..)
And they have to pretend to be others, they use fake name.

I remember a scene if that can help: Buffy need a bath, and Spike make her a surprise, he fill a large tub by heating a lot of bucket of water (you know like in "Little house on the prarie", yeah I know, I know....but I'm kind of desperate), to make her a bath, he installs it near the river I think, out of sight.

I thank in advance the generous soul that find me this fic

There are two possibilities here. Most likely, this is by uisge_beatha and is her permanent wip: Manifest Destiny. That sounds exactly like it. Unfortunately, it looks like she has blocked it on her Live Journal, which may be the only place it was posted. I'm sorry she lost interest and never finished it. It was a great story. That is the title/author, though, so you might try some of the archives in case it's just her LJ that she shut down.

Thank you! It's this one! That's super nice to found it for me. It's a pity that she doesn't have finished this story but I'm glad to be able to read it again.
You're adorable (and I'm in love with ALL your fics !! With a special mention for "Baby, you can drive my car")

LOL - Thank you. I'm glad you've enjoyed them. And you're welcome. I'm so pleased when I know the answer to these searches.