Signals II: Block Systems

On a railroad, lineside signals provide information to the person driving a train (the driver or engineer, depending on which country you live in; I’m going to use the word “operator”). This allows them to go faster than if they were limited to what could be seen directly. Trains are heavy and steel wheels on steel rail slide fairly easily, so it can take more than a mile (1.6 km) to stop a train moving at a reasonable speed.

Braking distance isn’t the only thing that affects train speed. At places where tracks diverge, or when changing tracks, a train may need to slow down due to the speed limit imposed by the turnout(s) being used. For this reason, signals used at places like this (one of several types of “interlockings”) get more complicated. As noted above, I’ll address that aspect in a future post.

Where trains don’t have a choice of direction, what controls speed are two things: unchanging limits imposed by equipment or track, and variable limits due to conditions ahead. Inherent limits are things the operator knows before boarding the train: the limits of the equipment and permanent speed limits imposed by track geometry (sharp turns, etc), and temporary limits (such as a limit imposed until a known problem can be fixed). Those limits may also be posted on signs, although this depends on the railway, and often the operator is required to memorize both the normal limits and any special limits in effect that day.

Block signals historically have worked to limit speed based solely on knowing how many block sections ahead of the train are clear, up to some maximum. The speed limit associated with a given indication is either encoded into the interpretation of the signal (e.g., “yellow means 30 mph”) or another detail the operator needs to memorize. The signals work by using electrical “track circuits”, which can also detect rails that break due to accident or environmental conditions (rails stretch and shrink as temperature changes, and sometimes they snap).

This makes block signals, usually, much simpler than interlocking signals. However, block signals adjacent to an interlocking may be a hybrid of the two, and able to display additional information relevant to the interlocking while still being part of the block. We’ll cover that aspect with interlockings, and today focus only on block signals away from interlockings. These are sometimes called “intermediate block signals”.

Fundamentally block signals provide an indication of the distance (in block sections) a train has before it must come to a halt. That can be “unlimited” (meaning longer than the worst-case braking distance) or some number of blocks. It’s not that simple of course.

Signals I: Development, Regulation and Use

For more than 150 years, signals beside the tracks have been used to provide guidance to the operators of trains. Originally this was a simple “stop” or “go” message, but over time it became more elaborate, and the signals themselves more complex. Today signals provide fairly detailed guidance that allows for efficient and safe operation. But how they do that varies a lot between railroads. Signals have also become specialized, with signals at stations, junctions and similar points (“interlocking signals”) behaving somewhat differently from signals along uninterrupted lengths of track that exist mainly to separate trains (“block signals”). There are also many other, more specialized, signals.

Signals used in Japan are both simpler than those used in many other places, and allow for some capabilities that others do not (or that they do using more complex methods). But they also have a lot in common with signals used elsewhere. That shouldn’t be a surprise, as Japanese practice originated, as did that of many other countries, in British practice of the late nineteenth and early twentieth centuries. However they were also influenced by North American practice (which itself originated from mid-to-late-nineteenth century British practice). And they created some things unique to themselves. But I think that to understand them, it helps to take a look at how signals are used on railways around the globe, particularly block signals, as Japan has streamlined their system by focusing on block functions.

I’m going to leave Japan for this post and wander the globe for a bit before I get back to explaining Japanese signals in a separate post. But I will include Japan in today’s discussion.

Compact Unitrack

was going to post something “soon”, and soon came sooner than expected. I went to a model train show today, and having Kato’s new compact track on the brain after working on my Unitrack pages last night, couldn’t resist picking some up. I bought a CV1 “Compact Oval” set, along with two R150 switches and a small pack of 150mm (6”) curves. This builds an oval with a short-cut inner curve that fits snugly on one end of my coffee table, with plenty of room for the Kato power pack.

A Short History of Transistor Throttles

My interest in the design of transistor-based DC throttles (aka Power Packs) for model railroading ended up causing me to pick up the DVD set of Model Railroader back issues (henceforth identified as MR). While US$200 seems like a lot, I think it was well worth it, if only to satisfy my curiosity. And it works out to less than US$0.30 per issue, so in a sense it’s a bargain. I also dug up a copy of Peter Thorne’s 1974 book Practical Electronic Projects for Model Railroaders (mine is the third edition of 1975), which has a number of throttle circuits, including one using an SCR. This book can go for rather high prices online, but I found mine at a train show last week for the cover price of US$3.50; quite the bargain.

Early-on electric model trains were run with car batteries (some early ones used AC motors with AC from a transformer instead), first apparently at 6 volts but by the 1930’s DC motors were apparently designed for 12 volts even before cars switched to the larger batteries, requiring two batteries placed in series (per MR August 1934 article on the use of DC power). DC at 12 volts was more than enough to run small motors, and early throttles were little more than a variable resistor (rheostat) to reduce voltage for slower speeds, and a Dual-Pole, Dual-Throw (DPDT) switch to reverse polarity for direction control. Often a “knife” switch would be used for the reverser, which could be left in a central “off” position to disconnect the throttle from the track.

But modelers weren’t very satisfied with these. DC didn’t allow for smooth low-speed operation, and “jackrabbit” starts with a minimum speed over 10 or even 20 scale miles per hour (16 - 32 kph) made for poor switching operations. Plus, modelers wanted to model the behavior of real trains, with simulated momentum and realistic braking action.

This led to designs for more sophisticated “throttles” and ever more complex designs as electronics technology improved. Some of the results did a fairly good job of replicating the real behavior of trains, right down to simulating the performance of air-brake systems similar to the one in the diagram at the top of this post. It’s possible some of this took place before the transistor was introduced; vacuum tubes could have been used for similar things. However, nobody appears to have published their experiences with these, so it seem likely that little or nothing was done until the transistor came along.

The development of the low-cost transistor in the late 1950’s made more complex throttles accessible to a hobbyist with a relatively minor amount of electronics skill and for a reasonable price, and the next decade was a time of rapid change, with evolution continuing into the 1970’s. By 1980, interests had shifted towards running multiple trains using command control systems (the precursors of DCC), although the roots of those went back further. And even in 1980 you could still buy rheostat throttles, although they were definitely behind the times by then. None of these technologies fully displaced the others. The transistor has in fact soldiered on into the era of digital controls, and you can still buy transistor throttles today that aren’t too different in principle from those designs of a half-century ago.


A Matter of Time

Railroads have always been concerned with time. Early ones used timetables alone to keep trains on the same track from colliding. This didn’t work very well, particularly given the accuracy of mid-1800‘s pocket watches and the lack of synchronized time sources, and many lives were lost. Signaling systems and other protection methods were gradually developed. But timetables continued to be important for all trains in scheduling the use of tracks even with other systems used for protection, and timetables were required for passenger trains, as trains from different places needed to coordinate their arrival and departure at interconnection points, so passengers could move smoothly from one to the next as part of a longer journey. Railroads gradually developed standards for time-keeping (and they’re responsible for the adoption of the standard time zones used in the U.S.), and also influenced the development of clocks and watches to provide accurate and synchronized time sources.

Japanese passenger trains today are famed for their obsessive adherence to schedules, with deviations measured in seconds, not minutes. So it only seems reasonable that a model railroad of a Japanese passenger line should operate to timetable (well, eventually,I need a yard/staging tracks before that will be much fun).

A Busy Day

It was a busy day in the village overlooking the Sumida River. A steady parade of trains rolled by on the embankment: commuter trains bringing workers to the city, resort trains taking vacationers away, and freights carrying commodities to and from the ports on Tōkyō Bay. But then, all days are busy on the railroads of Tōkyō.

Subway First Run

Sunday, 13 June 2010 was an historic day for Sumida Crossing. After the track was all cleaned and re-installed, and the wiring completed, it was time for the trains to take a run. The actual first loop was done by a “maintenance of way” train (actually an old Atlas B23 I was willing to sacrifice if I’d made some horrible wiring error). That done, I broke out the East-i E Inspection Train, and had it take a run to check out the pantograph clearance and general track usability. And I recorded it and made a short video.