There is an article in a 1948 issue of the Erie employee magazine that describes the earliest mass application of mainline radio in both passenger and freight service on that road; it goes into many of the details that were supposed to be 'new' such as the use of FM at FCC-determined 'railroad' frequencies with restricted interference potential, infrastructure for reliable "EE" communication, and use of handheld devices in yard communications. A historian of technology will rapidly appreciate that transistors becoming practical decades later only adds more reliability to an already well-established technological approach. In any case, you will find if you actually do the history that Erie used the "Radio Equipped" (with the bolts) and Lackawanna just the word "Radio" before the EL merger, and the EL famously adopting the familiar form of the slogan shortly afterward, certainly before the widespread advent of transistorized equipment on those railroads. And that was the question being asked.
I find it interesting in 1960 that the Canadian system was reserving reassigned frequencies A, B, and C for 'satellite' communications that would reduce much of the problem with lineside and route interference with both EE and fixed-point transception... again at a time that all commercial equipment was essentially tube-based. It would be interesting to see what a contemporary railroad satellite receiver would look like ... or the AAR plans to orbit the necessary capacity. Atomic locomotives look a bit simple by comparison... but don't laugh; they'd promoted the functional atomic switchlamp only a few years earlier.
It might be amusing to speculate on what would be required to make a ruggedized tube chassis capable of modern 6.25KHz narrowband performance. Careful 'crystal-oven' temperature management, for one thing. Don't laugh: the Russians used tubes in key supersonic-fighter systems into the Seventies, largely because they were more tolerant of nearby electromagnetic pulse in a combat environment.
Meanwhile, for more technical detail on early train radio, there was a 'picture of the day' on the Classic Trains site in March 2010 showing an original MoPac installation. As I recall this had a small steam turbine doing the work of the motor in a dynamotor to supply the appropriate voltages for the radio set. It would be interesting to see this modeled, and the wiring described, in the same way we see auxiliary systems like feedwater heaters and lubricator systems described for modeling accuracy.
I think you'd be safe using the "RADIO EQUIPPED" lettering on c.1954 equipment. One thing to keep in mind is it was used when radio equipped engines and cabooses were something new...kinda like how c.1900 some railroads put large "AIR BRAKE" lettering on cars equipped with air brakes. They quit doing it once all cars had air brakes. I don't think there are any BNSF engines with "RADIO EQUIPPED" on them, since by the 1990's it was assumed all engines had radios.
OvermodThe introduction of solid-state radios was significant for practical motor-carrier use of two-way radios...SNIP...But that is not the era this thread is concerned with.
I'm not so sure about the era being as set by the question. Certainly, there were radio-equipped units out on the road earlier. Did they have the familiar "Radio Equpipped" logo seems tto be the question.
Given the Erie made the claim to being the first to be radio equipped, the example I've seen of ithis early logo suggests that it as not yet in the more familar form it was widely used in later. See the way the logo is on this Erie caboose (a restoration, but we can probably assume the logo depicted was accurate). It simply says "Radio" with the lighting bolts on either side - and in huge lettering compared to the later widely used, more petite "Radio Equipped" with small lighning bolts. This suggests it was derived from the original Erie depiction.
http://www.whippanyrailwaymuseum.net/41-facilities/museum-equipment/212-erie-lackawanna-c-177-caboose
As a historian of technology, it's often more important to determine when something became widely used, as this is often more culturally and socially significant than when it was first used, whether it's the technology in the radio or the logo that's the subject of the question, as that shows its broad adoption by industry. First use is important to document for patent protection, bragging rights, and maybe a Nobel Prize, but it's the pace and spread that happenes next that is more significant in determining the impact of innovation.
I agree that the early use of "Radio Equipped" often was on units using older technology radios. To the public, it was the fact that it was a radio, not the type of radio. That said, before WWII this was the result of adoption of radio in a few specialized circumstances, where traffic was haevy enough to rate its expense and maintenance headaches. True, just using transistors didn't make a radio entirely reliable, but it certainly increased the possibility and mean time between failures versus even ruggedized tube technlogy adopted from militay use. That's what led to the widespread adoption of radio.
Add-on: Found this link with info on the RR radio service. Down past that, there's some brief notes on folks with RR connections returning from serving in the military RR service. Turns out there were several from the Erie and the D&RGW, which reinforces your argument about the war's effect on spreading desire for radio service among RR management.
https://books.google.com/books?id=6o3mAAAAMAAJ&pg=PA849&lpg=PA849&dq=%22radio+equipped%22+logo+railroad&source=bl&ots=O_vRqoZHph&sig=suvx4Z9Gn1Qw1PhpdojyQhBM_40&hl=en&sa=X&ved=0ahUKEwjftNz_qfPYAhUl0YMKHavxAZIQ6AEISzAH#v=onepage&q=%22radio%20equipped%22%20logo%20railroad&f=false
Mike Lehman
Urbana, IL
Oh, for heaven's sake, Bardeen's transistor was point-contact, and in germanium. If you study the history of transistor/transistron development even a little you will see just how shock, moisture, and vibration-sensitive all the commercial devices through the mid-Fifties were. (I could get into the relative lack of application development at Bell Labs vs., say, Sarnoff in Princeton at about this time, but I suppose you're familiar with that from your adviser; I have certainly heard about it firsthand from many who were there.)
The introduction of solid-state radios was significant for practical motor-carrier use of two-way radios. And there is no question that by the time appropriately ruggedized solid-state transceivers were cost-effective they represented a much better and more reliable solution than anything tube-based, and much of the discussion of practical reliability of radio for operations purposes finds adoption delayed even until the Seventies in many respects. But that is not the era this thread is concerned with.
But I stand by most of the early installations meriting 'Radio Equipped' on locomotives reflecting older radio technologies, probably as I said benefiting from military applications both in design and in cost-effective parts availability.
OvermodI think the primary argument Mike advanced is the correct one: that the combination of wartime technical advance and 'surplus' availability of parts and engineering led to low-cost availability of suitably rugged systems.
A little more background on my argument, as radio engineering was not one of my academic focuses. My doctoral adviser, Lillian Hoddeson, wrote a biography of John Bardeen, "True Genius: The Life and Science of John Bardeen: The Only Winner of Two Nobel Prizes in Physics," one of the three inventors of the transistor who won the Nobel Prize for it. Both were faculty here at the University of Illinois. She also did extensive research over the years into the research culture at Bell Labs, where it was invented.
While radio was recognized early on as a very useful tool by the railroads, it wasn't until the transistor arrived that radio became functionally and economically viable throughout the industry. An interesting view is provided by these circa 1960 FCC documents on Canadian RR radio adoption, which was linked with similar US efforts by the need to coordinate across the border and between different railroads. More here (PDF file):
https://transition.fcc.gov/ib/sand/agree/files/can-nb/can-rr.pdf
A 1945 article on adoption of federal regs specific to RR radio use:
https://books.google.com/books?id=6o3mAAAAMAAJ&pg=PA849&lpg=PA849&dq=railroad+radio+service&source=bl&ots=O_vRoh6Goh&sig=stQ5EABVLs1UZwVD4KsHxDLpyLY&hl=en&sa=X&ved=0ahUKEwjg2rCl3evYAhVM2IMKHWOeADE4ChDoAQhPMAY#v=onepage&q=railroad%20radio%20service&f=false
Adoption of the familiar "Radio Equipped" signage was most likely a AAR initiative, but that requires further research to pin down more specifically.
An earlier thread on this topic:
http://cs.trains.com/mrr/f/13/t/243253.aspx
Lackawanna implemented radio extensively by the 1920s, promoting the benefits in their advertising; I believe ruggedized tubes in train-control and cab-signal equipment were used not later than the early Thirties. Personally I think the Depression, not failures in 'radio engineering', was the likely reason for low adoption of the technology. I think the primary argument Mike advanced is the correct one: that the combination of wartime technical advance and 'surplus' availability of parts and engineering led to low-cost availability of suitably rugged systems. If I recall correctly this was brought up in the trade press at the time (I only remember this peripherally while researching other subjects) and some fairly 'hard dates' on adoption of the specific slogan mentioned in the OP on some specific roads could be found by browsing issues of those publications.
References are made to the FCC setting up the railroad frequency bandplan in 1945. The Pennsy and a few others experimented with various methods before WWII, but wartime advances in electrical and radio equipment facilitated such uses. The problem was that it was tubes that ran thngs and they tended to prefer a static, low vibration environemnt, something pretty rare on the railroad.
It was the inventon of the rugged and versatile transistor that only bore fruit after the war with the first experimental transistor in 1947 and the first production transistor in 1951 that made widespread use of radio in RR ops possible, as it did in many other applications.
About what time did the words "RADIO EQUIPPED" begin showing up on locomotives and cabooses?
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I think 1954 is too soon, but I have a set of decals that looks really cool that I want an excuse to use.
Thanks for helping.
-Kevin
Living the dream.