Safetran's guarantee of chromicity and beam pattern is a relatively recent development (within a couple of years or so).

A.A.R. (now A.R.E.M.A.) chromicity specs are important to minimize the chance that someone with normal color vision will mistake one color for another, especially when atmospheric color distortion is present. That requires a very narrow band color of light; which is much easier to accomplish with LEDs. With proper chemical doping during manufacture, any color LED can be produced. And LEDs are naturally very narrow band devices; unlike the incandescent bulb they replace. So this is an easy spec to meet with LEDs.

With proper lensing, any pinpoint light source can easily be focused into any beam shape desired. Neither of these present much of a problem.

However, what was a big issue holding up the parade, was a seemingly obscure F.R.A. requirement that signaling and train control devices shall not easily respond to accidental crosses (shorts) and grounds. Most safety related circuits were thus low input impedance devices, which made it harder for a typical cross or ground to unintentionally activate the device.

In a searchlight signal, which used a single whitish bulb, color was created by mechanically passing a color filter in front of the bulb. thus unintentionally lighting the bulb was of no concern. Besides, most railroad signal bulbs would not light up with with less than @70% voltage anyway. Thus A very hard short or ground would be needed to supply the needed current to light the bulb.

However, with color light signals it a whole other matter - you could say a "horse of another color" - hehe (sorry). Not accidentally lighting the wrong bulb is a safety critical concern on a color light signal.

Now enters the F.R.A. again. The Title 49 of the C.F.R. has been updated to require that the vendor of any new electronic signal or train control device, which replaces any existing non-electronic device, must prove that the new device is equal or better in safety than the original. To accomplish this in the case of signal LEDs, their vendors had to prove that the same (worst case) ground that would not falsely light an incandescent signal bulb, would also not falsely light the LED bulb.

The manufacturers of the original highway traffic signal LEDs didn't even consider minimum light-up current in their designs, and so could not prove equal safety. But as long standing railroad vendors, such as safetran, get into the LED signal market, they are of course taking all railroad requirements into account.

Note that the railroad industry has always been held to a much higher standard than the highway departments. Indeed, we often give our engineering counterparts at the highway bureau a joking hard time about that - how we'd be put in Leavenworth if we did what they sometimes get away with.

As I said before. these issues with LED signals will eventually resolve. I have even heard now of putting heating elements in the signal heads, which can be turned on when needed to melt snow! The LED signal is the future.


Hope that helps

OPB