If you are a student of modern Physics, and are thus acquainted with the theories of Mr. Heisenberg, then you may or may not disagree with me when I tell you that two objects may not occupy the same space at the same time. The statement may or may not be true for subatomic particles, but on the real-world scale, it is most certainly true. It is so true, in fact, that the incident which is most likely to get people hurt on a roller coaster is not a derailment, passenger ejection, or structural collapse. The incident most likely to get the most people hurt is a collision of two trains. Accordingly, much effort goes into making sure this does not happen.
The means for doing this is, as you might have guessed Footnote 1, a block system. Some form of block system is employed on every roller coaster that runs more than one train. Here is how it works:
The entire track, from station to station, is divided into logical segments, called (surprise!) blocks. A block is a segment of track on which a train may be reliably and independently stopped. It stands to reason, then, that each block will contain either a block brake or a lift. The block is then operated in accordance with The Rule:
Thou shalt not allow more than one train to occupy any block, at any time, for any reason, lest thy trains collide resulting in much wailing and gnashing of teeth.
A very few coasters have only one block. The High Speed Thrill Coaster at Knoebel's is one example. On that particular coaster, the lift mechanism extends through the station, leaving only one zone where the train can be reliably stopped, specifically somewhere between the uptrack end of the station and top of the lift.
Most coasters, even those which only run one train, have at least two blocks. Typically, the first extends from the end of the station platform just beyond the brakes to the top of the lift; the second extends from the top of the lift to the end of the station brakes. This gives two places where the train can be stopped, but that still isn't enough to allow for two-train operation. You see, while we can detect when the train enters or leaves the block, we cannot be certain exactly where the train is. This means that to insure safety, we must religiously obey The Rule. But when you consider that coaster trains have some length, it stands to reason that at times the train must be in two blocks at once. The Rule permits this...while each block is restricted to only one train, the trains are not restricted to single blocks. It follows, then, that in order to run two trains, a third block is required.
On most coasters, this is easy to accomplish, as there is normally an approach trim brake just behind the unloading platform. All that is needed is the ability to stop the train on this trim brake if the station is not clear. In many cases, the train should never stop on the third block brake anyway , but the brake has to be there in case there is a delay in the station.
It isn't much of a stretch to see that in theory, only a fourth block is necessary to run a third train, and in fact for any number of trains, the coaster needs to have at least one more block than it has operating trains. In practice the margin is frequently larger than that, for instance, I have seen three-train coasters with as many as six blocks.
These days, at least some degree of automation is required to run more than one train. It need not be complicated, though; it needs only be elaborate enough to insure that The Rule is followed. Let us begin by considering a two-train coaster operated entirely by hand, such as Kennywood's Jack Rabbit.
Only the simplest of automation is used on the Jack Rabbit, but with trained operators, it is enough. The ride has three blocks. One block is the loading station, another is the first half of the ride to the top of the lift, another is the second half of the ride leading up to the unloading station. On Jack Rabbit, there are two brakes which are normally applied and released by the operators. If the operator does nothing, the train will stop on the unload brake. The operator determines when to roll the train on down to the loading station. In the loading station there is an optical switch; there is a similar switch on the final turnaround before the train returns to the station. If both switches are tripped, a bell rings, alerting the operator that he needs to get the loading station clear before the other train comes back. The timing is important so that the train returning to the unload station can be moved down to the loading station before the train just dispatched clears the lift...otherwise, a train has to be stopped on the lift in order to avoid a violation of The Rule.
These days, most roller coasters operate with a bit more sophistication than that. Many use electromechanical relays to open and close pneumatic brakes to start and stop the trains. The concept is exactly the same, but the safety system is automated. When the train passes out of a block, the brake in that block is set; when the next block is clear, its brake is set and the brake behind it sets. These brakes might open and close all day and never stop a train. But should a potential block violation occur, the closed brake will stop the train. By adding one more switch near the top of the lift, the lift can be used for blocking but only stopped if the next block is occupied AND a train is nearing the top of the lift. Sensors and relays can even be used to detect other situations, for instance preventing a dispatch if the transfer switch is open.
Relay logic, relay-based control systems are still used on new coasters. Vekoma, for instance, uses relay logic for the block system on their standard Suspended Looping Coaster, and Mack advertises that their Wild Mouse is available with either a computer or with a relay system. Relay systems have certain advantages, most notably that in addition to being proven, reliable technology (Harry Traver used a relay control system on the Riverview Bobs in 1924), it is technologically simple enough that any electrician with a voltmeter can easily troubleshoot and maintain the system. The problem is that the relays are electro-mechanical devices and will sometimes stick or burn out.
To take the ride controls a step further, the logic relays are replaced with a Programmable Logic Controller (PLC). The PLC is a special-purpose computer which simply takes real-world signals, makes decisions based on those signals, and produces output signals based on those decisions. In its most basic form, a PLC program operates in exactly the same way as relay logic. But the PLC is more flexible than a bank of relays. For instance, a PLC can easily be equipped with a switch to count brake fins as a train goes by so that it "knows" all the cars are accounted for. It can count lap bar release pedals so that it can detect a bar left open. It can be connected to an anemometer to measure the wind speed and set brake pressures accordingly. The PLC can even detect train speed at various points on the course and monitor the ride's performance. These days, most roller coasters use PLCs to control the block system, and the rule remains exactly the same as always: No more than one train may occupy any block on the ride at any time. The difference is that the PLC has the flexibility...because it is programmable and because it can do advanced processing on its inputs...to also automate and optimize other functions on the ride, and to monitor the ride's critical systems. The PLC has made ride controls idiot-resistant, and simplified the process of diagnosing system faults.
Footnote 1: ...given the title of this document...
I'd like to encourage comments, questions, corrections, and other insights on this subject. Send me your comments and I will try to address them here.--Dave Althoff, Jr.
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