Is the tag amtrak redundant if posts are already tagged with trains and united-states?
If no, should I tag relevant questions with all three tags, or only with amtrak — probably this implies trains and united-states?
If yes, is it technically possible to make amtrak synonymous to the combination of trains + united-states?
There are about two dozen regional/commuter rail systems in the U.S. that are not operated by Amtrak, but also would never be confused with urban rapid transit systems. They are not designed for long distance travel by tourists, but a backpacker could make the journey from New London CT all the way down to Newark DE (about 250 miles) entirely on commuter train services for about $50 (plus a walk between Grand Central Station and Penn Station in New York), as opposed to $127 on Amtrak.
In some cases they share facilities or lease rail from Amtrak, but to the consumer they have no other relationship— you can't go to a Metra agent and ask whether the Empire Builder is departing on time, and you can't go to an Amtrak agent at Washington Union Station and get a VRE pass. So while we wouldn't need a tag for each of those systems, it wouldn't be correct to tag them as Amtrak, either.
Is the amtrak tag redundant if posts are already tagged with trains and usa?
No - see @choster's answer. (The tag is presently in use for 43 Open questions.)
If no, should I tag relevant questions with all three tags, or only with amtrak — probably this implies trains and usa?
Only with amtrak (and others as appropriate). As you mentioned, amtrak already implies trains and usa. @Ankur Banerjee♦'s advice is:
use the most specific tags for the question.
This is advisable in part, because of the five tag limit per question, to leave room for other more meaningful relevant tags, such as train-stations, refunds online-resources etc.
If yes, is it technically possible to make amtrak synonymous to the combination of trains + usa?
Technically Yes, but very far from being practical or ever being implemented.
I think the people looking for questions tagged either amtrak, trains or united-states should find questions relating to amtrak, so I suppose it implies questions should be tagged the three.
But I agree, it seems weird to have all these, it looks like too many.
Are synonyms one-way or bidirectional? In other words, is there technically a way to "produce" the tags trains and united-states when amtrak is used, but not the other way around? If so, I would suggest this. If not, let's just tag the three.
I feel amtrak should be made a synonym of trains. Thoughts? Train companies usually have a monopoly on whatever routes they run, and even if they don't (such as in the UK, where the connecting trains between two points may be from different companies), the difference usually doesn't matter.
