Mexico City

I took the Inter-terminal train on May 13. You can just show your boarding pass and walk on with no fee. It is mostly standup with a few seats for elderly and people with disabilities. I was bemused to see signs asking people to sit on the seats due to covid, so we stood close together holding the handgrips.

I was supposed to have a same day connection, but the airline messed up on a matter of documentation. The airline acknowledged its error, but by that the flight had left and I had to overnight at the Camino Real.

It is years since I have been to DF. I remembered that the air was thin, but I had forgotten how thin. I am accustomed to doing 8-16 kilometres of brisk walking (eg. 60-80% of maximum heart rate) every day. I was getting exhausted just walking around the airport terminals.

The 2 people I asked, both of whom had a uniform, seemed to I dictate I needed a 'reservation' to take the tram

No reservation needed. Just a valid boarding pass for the terminal you are going to.

Ok
Without beating the issue to death
I didn’t have a boarding pass for T2
I was going to see if the izzzsleep hotel had availability after the one at T1 was full up.
Is it possible the people who told me a reservation was needed meant I needed a flight out and not a ‘reservation’ fir the tram?
Seems unlikely

They want to make sure you have business going on the tram. [a boarding pass] You miss understood about a "reservation" I have even explained [successfully] that I need to go to T-2 to get my boarding pass, and just arrived at T-1. Again there is no reservation system. Remember the tram is outside the airport secured area, this is why they are checking for boarding passes.