How do block signals work?

I was watching trains at Bound Brook the other day and got to thinking about block signals. Can anyone tell me, in plain English, how they function? I'm assuming one rail is a + and the other acts as - but can't figure it out.

One theory is that the left rail is + with a current and when the metal wheels/axels touch the - rail it completes the circuit and power flows thru the red light bulb. The problem is the current loop is before the bulb.

So theory #2 is that one rail is + and one is -, when the metal wheels/aels touch the - rail it becomes +, you then have two + lines causing a short circuit. But how does a short cause a red light to illuminate?

The circuit is complete when the train is not occupying the block, when the train occupies the block it shorts the circuit. At one end you have a relay, at the other end you have a very low voltage battery. The relay on the end with the proceed indication is held closed by the voltage from the battery. Once a train approaches it shorts the circuit and thus opens the relay, dropping the signal to a stop indication. It cant get any simpler than that. It just gets more complicated as the more you wanna know about it!

AntNJTrainSet is conceptually correct, but he uses the term "block" to describe what is called a "track circuit."

The best way to look at this is by the theory of things being "fail safe." Not meaning "safe from failure" but rather "when there is a failure, things are safe."

By putting voltage on each rail at one end of the track circuit and connecting a relay to the rails at the opposite end of the track circuit it becomes "fail safe" because not only will a train shunting the circuit cause the relay to drop (de-energize), but so will a broken wire, broken rail or loss of power.

Since this is a question that is not limited to New Jersey topics, I will be moving it to the General Discussion: Railroad Operations Forum.

-otto-

i recall been on a dmu in england , where i could see a pair of jumper leads hanging behind the driver . these were to short the rails out in the event of a breakdown , as it was considered the light wieght of the dmu might not be enough to make the ciruit at standstill . if the rails / wheels were dirty i guess .

David Benton wrote:i recall been on a dmu in england , where i could see a pair of jumper leads hanging behind the driver . these were to short the rails out in the event of a breakdown , as it was considered the light wieght of the dmu might not be enough to make the ciruit at standstill . if the rails / wheels were dirty i guess .

This is true, here in the states we just get protection from the dispatcher. A single car MU (arrows, DMUs etc) is usually too light to continously shunt circuits so they have to be reported clear of locations. Sometimes a single lite diesel or electric locomotive can have this problem too. Its even worse with rusty wheels/rusty rail.

AntNJTrainSet wrote:A single car MU (arrows, DMUs etc) is usually too light to continously shunt circuits so they have to be reported clear of locations. Sometimes a single lite diesel or electric locomotive can have this problem too. Its even worse with rusty wheels/rusty rail.

Which is why there's a capacitor across track relays.

a capacitor would help to smooth out an intemittent signal , but would be of no use if there was no signal . i.e a stationary vechicle failed to short out the track .
Do you what the voltage in the circuit is , and what sort of amperage is involved ?

I know the voltage is very low, the amperage cant be up there either, its just a hold current. The voltage i believe is 12v. Most signaling systems run on 12v automotive lightbulbs, and if i remember correctly that is the voltage of the batteries and the chargers.... someone correct me if im wrong.

Another thing thrown into the mess is electric traction. Theres a thing called an Impedence Bond thats made across track signaling circuits. The Impedence bond allows for the 12kv/25kv/600VDC return circuit to go back to the substation from electric trains across an isolated joint in the rails for the signaling system, its a high enough resistance that it wont allow the signal circuit to pass but it will allow the return from the electric trains to pass.

The track circuit battery is separate from the signal lamp battery and is lower voltage, typically 2 or 3 cells of 1.2V each in series so voltage is 4 V or less. Track realys operate in the general range on the order of 100 milliamps for pickup and 50 milliamps for dropout.

The battery is in series with a resistance to limit current when the circuit is shunted by a train. FRA rules require the circuit to detect a shunt resistance of 0.06 Ohm anywhere in the circuit.

This is a very simple generic description of a basic DC track circuit. AC and high frequency circuits are also used in some applications, particularly when there is AC or DC traction power in use.

The basic princple of any track circuit is an energy source at one end and a detector at the other end which will sense either a break or shunt in the circuit. Modern crossing protection circuits may use high frequency AC circuits and detect phase shift to determine motion and/or speed of a train shunting the circuit.

The main challenge in the design and maintenance of any track circuit is balancing the ability to detect an actual shunt from a train vs leakage between the rails from wet and contaminated ties and ballast.

Cool. Thanks for the corrections/info clearblock.