>>These 2 tracks look identical to me. The metal one is for glass, the plastic one is for lightweight.
Both of those door tracks can be used with glass. I have a plastic track that I've been running 1/4" glass on now for almost 3 years. I even had a sand substrate in the cage that my Bearded Dragon kicked into the track on a regular basis. Even with all the sand in the track it has held up very well. I did eventually shift to a less abrasive substrate, however.
Maintain the polished edge on your glass and occasionaly spray the track with silicone or Teflon based lubricant. I will last for years.
z..I know acrylic is lightweight but will it hold an adult boa, but what if I use plexiglass, say a 6'x2' door, is it going to weigh a ton? If I use acrylic, it would save me $70 off the start on tracks (2x$45 vs 2x$11).
I'm not sure what you're asking. Plexiglass is a type of acrylic, they are not different.
Again, the plastic track is fine with glass. Dozens of us here along on this forum have used it that way. If you go through Kingsnake's Photo Gallery you can find countless cages with the Knap and Vogt plastic glass track. You can't miss it since it's an obvious orange color.
Also, do you need two tracks because you need two cages or do you think you need to buy the upper and lower track separately for one cage? I ask only because I know people who have purchased two thinking that's what they needed to do.
Regarding whether to use sliding or fold-down doors, use what you prefer.
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Current snakes:
1.1 Gonyosoma oxycephala - (Silver/Yellow)
3.4 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)