Upon reading Ready Player One, I was reminded of a novel I read some time ago. I thought it might be Alan Dean Foster but nothing on his bibliography looks like that.
Two main characters, unknown to each other in real life and geographically distinct, are in a massive online game system that uses immersive VR with haptic technology. The training scenarios feature the set turning to cardboard before starting and after finishing.
A fighting simulation based on Beirut features into the plot, as does a special simulation in the form of a train.
Somehow they are stuck and can’t exit. I seem to recall that one is a diabetic and would die fairly soon from this predicament.
I think it was decades old. Could be as far back as 1980’s.
It’s not the same as this one.