I merged rail and railroad on May 17 after a question (now deleted) was asked using both tags at once. The author of the question was a new user without the "create tags" privilege, so I decided to investigate the apparent redundancy between pre-existing tags. Since they were being used identically and had only about a dozen questions between them, it seemed like the definition of an uncontroversial merge case.
Light rail systems are neither described as railroads (in the infrastructure sense of the term) nor operated by railroads (in the business entity sense of the term). "Rail" here is not an abbreviation of "railroad" in either of the above senses; instead, it refers to a mode of transportation. Rather than create an entirely new tag for one light rail question, I preferred the slightly more general term for the merge target.
Note that even the Federal Railway Administration of the US DOT, which has "railroad" in its name, prefers "rail" when referring to the mode of transportation; see this press release, for example:
The FAST Act authorizes \$305 billion over fiscal years 2016 through
2020 for the Department of Transportation’s rail, highway, motor
vehicle safety, public transportation, motor carrier safety, hazardous
materials safety, research, technology and statistics programs. The
FAST Act also marks the first time intercity passenger rail
programs have been included in a comprehensive, multimodal surface
transportation authorization bill, authorizing more than $10 billion
for intercity passenger and freight rail grants.
If you think there's a better way to categorize these questions, feel free to write up a brief proposal. Moderators can delete tag synonyms if that's what the community wants. But when a tag has such a small number of questions, we tend to prefer the simplest and least-fragmented tag convention.