C-H:
re. heat resistant plastic lampholders. I'm more of an old fashioned guy. I would prefer porcelain. It doesn't deteriorate at all with high heat.
However if you're going to use plastic....the best plastic for a lampholder is thermo-set phenolic (Bakelite) which is that brown plastic that was so ubiquitous 50+ years ago.
It's still widely used here in the USA and also in Asia for wiring devices. I wouldnt recommend melamine (white) and certainly NOT urea.
Both of these deteriorate and discolor under high heat. Bakelite does not.
---
Bjarney:
Apparently the reason for the double filament is that if one of the filaments burns out, the second one takes over. That way the bulb lasts twice as long and you don't have the problem of a disabled signal light in a tunnel holding up train service. The second filament holds up until the tunnel maintenance crews make their normal inspection rounds I guess.
And also crews don't have to go down into those tunnels to replace bulbs that often, which from what I've seen here in New York, is no piece of cake (what with the rats, water, 600-volts of electricty, crack-heads and heroin addicts and that ever present soot and dust that gets in your lungs and made my silver ring go BLACK when I walked into one of them).
