The new signals being placed into service by all freight RR's are tri-lights with lenses placed vertically. Why must existing similar signals, used for years by UP, CSX, BNSF, etc., be replaced?

new signals may be using new LED technology vs bulbs. Since going to the new technologies the old signal technology may not be compatible or usefully compatible (I know I know I dibble in electronics theres always an adaptative possibility)

Like the steam engine the older signals may become obsolete (in the usual company terms)
so max you compatibility abilities to the new technology, change to the new signals and yer safe.

Like so many signals on the NS (N&W position lights) being replaced with new signals, and with so many trains crossing railroad borders it keeps some signal rules compatible between railroads. With all the mergers we have some super railroads with differing signals in their territories from merged railroads, so switching to one universal signal that fits the full system makes sense.

I have no idea if say the NS has all signals switched over, its an ongoing process.

LED's are a different lighting demon, most light eminates from a single point than a broader field most lamps have, so lenses may need a different design. often to make up for the broader field lamps do multiple LEDS are used to broaden the lighting field.

I have to admit that I really dislike the new hooded LED "Darth Vader" signals. I understand why the railroads have to use them, but that is one more reason why we are into railroad preservation. We can use and/or display the old signals, all of which have "character" unlike the "antiseptic" new ones out there. Hopefully it will be a long time before we see LED signals at a railroad museum.

Les

Here in Roanoke we've bee lucky enough to save a fair amount of historic signals, both N&W and VGN, for future display

The reason that railroads are replacing existing, and often antiquated signaling with a newer and more standardized version isn't exactly because of PTC. I can explain...

To put it simply, if you ran a fleet of cars or various makes, models and years, and the government required you to install new computer, or some sort, into the existing cars' wiring....would you (A) spend thousands of man hours to customize every single car with this computer, with mixed results and mixed equipment, or (B) seize this as an opportunity to standardize your fleet, and buy all new equipment that comes pre packaged to add this computer as a, more or less, plug and play component?

Option B is what most railroads are doing. It allows many sites with aging cable, equipment, enclosures, batteries, etc, to be upgraded fresh...some for the first time in 50 or so years. If pole line is being used, as well as a traditional relay system, this can all be upgraded to Electro Code or Microtrax, allowing more standardized troubleshooting. Communication systems are being upgraded as well...new towers, coax and radios.

Last, but the most 'foamer visible component'; the signals. There is no reason that existing signals can't be reused...in some cases, they are being reused. However, aging signal masts and rotten foundations, as well as wooden plank floors on cantilevers and bridges, are every maintainer's nightmare when troubleshooting an issue at 2:45am on a rainy night. Also, searchlight mechanisms have to be 'rocked' or tested every six months, and formal testing has to be completed every 2 years...any maintenance foreman who has to shuffle all of this paperwork on, potentially, hundreds of mechanisms, will love to see them hit the scrap bin.

It's much easier to install and cutover a signal system by using a completly new signal. There are plenty of photos of new signal installs on NS, CSX, UP, etc, of 'Vader' signals replacing modern 'vader' signals. This is the reason why.

There is no reason why you couldn't implement PTC on any existing installation that is out there today. However, you will have to install a tower, radio, backup batteries and some computer logic, which generally requires a enclosure of some kind...say, a 4x4 or 5x5 bungalow. At most intermediate locations, by the time you go in the field, install this bungalow, raise the tower, and wire everything up...you might as well run new cable to your track and signals, throw a Electro Code box in the same case, and put up a few signals, too. It's not much work, nor is it rocket science. This will save a wireman hours in the field from trying to install PTC equipment to interface with the existing installation, when the same wireman can work the same amount of time, and you end up with all new equipment...standardized across the railroad.

As far as LED technology is concerned, BNSF is still hanging onto incandescent lamps in all new installs. Modern signaling systems utilize a lamp-out technology which allows the lamp driver on the signal systems' computer to sense if a lamp is drawing current, or not. If the signal's vital control tells it to display a green and the green bulb is burned out, the lamp driver will know this, and downgrade the signal to an approach. This way, a train does not have to stop and procede for a dark signal.

One of the problems with a LED is that a failed LED with a power supply, can still draw current without displaying light. This is why LED technology in signals has only taken root within the past 10 or so years. Some, like BN, are still hanging onto traditional lamps. They are easier and faster to change out then LED units, and are cheaper, too. Modern LED modules utilize special sensors that can sense if the unit is actually producing light or not, and after extensive research and testing, most railroads have accepted this as their standard...but, it wasn't overnight by any means.

The 1920's was the era of the Semaphores. 1960's was the Searchlights. 2000's is the color light. 40 years from now?....

I feel pretty lucky to have gotten to run under both semaphores AND searchlights in the last 10 years on the mains of Kentucky and Indiana.
Joe

I'm just glad I caught the tail-end of B&O CPL's.

Some railroads complain that LEDs won't melt snow that
can stick to the lens cover, so they stay with hot
incandescent bulbs.

wcturuboat,
Thanks for the detailed and clear explanation.

So far as the technical requirements of PTC go, a signal is a signal is a signal.....they're all the same so far as the logic goes.

But as hinted at above, the searchlight mechanism has fallen out of favor in recent decades. That because it has a documented history of unsafe failures. If it remains in one position for a very long time, it may "stick" there when called upon to change position. The shift is to signals with no moving parts, whether incandescent or LED.

In 50 years in signal engineering I've learned that "new" is not necessarily "wonderful." This is amply illustrated in the difficulty of providing fail-safe "light-out" protection for LED signals.

Mechanical interlockings still work just fine too.

Does PTC track automatically meet the requirements for greater-than-79 MPH operation if the track itself is in sufficiently good condition?

Not necessarily; signal spacing is one determinant, there must be others.