word usage - Must "Eldest" Always Apply To People?

If you have a collection of things that are related to one another, can you use "eldest" to denote the oldest, or should that term only be used with respect to people?

Another question on this site: What's the difference between “eldest” and “oldest”?, partly explains when "eldest" may be used, but doesn't clarify if it must always pertain to a person or persons, or whether "eldest" can be also applied to things which are not people.

To give an example, I wrote in a comment elsewhere today:

As a lifelong Phone Phreak, I can only look back on what was once the Bell System with fond memories. It, and the Bell operating companies that Verizon originated from, are as much a thing of the past as the Pullman Company that manufactured our eldest Red Line trains.

I chose to use "eldest" rather than "oldest", because I wanted to express a certain respect for something that was built in a previous era, yet has endured longer than expected. I wanted to suggest the sort of respect one would associate with an elderly person who had reached a ripe old age.

To explain, here in Boston, there is a family with three or four models of Red Line trains. They're related to one another, but from different generations. The oldest were manufactured in 1969 by a once great American company whose name was synonymous with train car (and luggage). These trains have far exceeded their expected lifetime, but are still in service every day.

Was it grammatically correct to use "eldest" in this context?

Answer

Thanks to anthropomorphic personification you can treat anything like a person. Just think of it as a person. For example, my computer sometimes gets moody and won't talk to me until I give her a little break from work.

That said, What's the difference between "eldest" and "oldest"? In a nutshell it's that the set of things being compared in age must be related in some way for eldest to have meaning. If it could be said that your four models of Red Line trains are from the same family (and you already have) then referring to one as the eldest is simply keeping the family metaphor going.

So, it's not literally true, but it is grammatically correct. Grammar doesn't care about literal truth.