Nah I'm pretty sure Xen and everything after that takes place on the alien planet and if I remember correctly you briefly get this cool jumping suit and it's fun at the start...but then you into this really lame teleportation jumps in which you have to try over and over again and the gravity was screwed up and such I was constantly running out of ammoI absolutly hated On A Rail and Power Up. IMO, they are the worst part of HL1 while "We've Got Hostiles", "Forget About Freeman!" and Interloper are the best parts.
Everything in the actual facility was awesome EXCEPT for that one part with the giant tentacles that you couldn't kill and had to somehow dodge