Where can this FAQ be found on an official site?
There doesn't seem to be any information about why the FAQ is being hosted at the fan site, and not at Mayfair Games itself, but there are several circumstantial pieces of evidence.


Mayfair Games don't have a FAQ for Empire Builder.
Mayfair Games links to the fan site (implying endorsement) from their official Empire Builder page.
That site is also the developer of the official computer version (i.e. licensed from Mayfair Games).
The resources for the game are comprehensive. It's unlikely they would make the FAQ up and then claim it was from the publisher.
There is another FAQ on the same site which indicates it is the set of rules by which the largest tournaments are played. That argues that this site is indeed the keeper of the de-facto most "official" FAQ.


For completeness, I did do some digging in the Internet Archive for a FAQ from 2008 hosted at Mayfair Games, but found nothing.

Interesting question. The linked FAQs look detailed and helpful. Now I'm curious as to why you require proof of provenance.