Timetable, Train Order, Block and Block Signal

Posted by PITHOCRATES - November 20th, 2013

Technology 101

The Preferred Method of avoiding Train Collisions is not being where another Train Is

Automobiles are relatively light and nimble.  It doesn’t take much energy to get them moving.  And it doesn’t take much energy to stop them.  A person only needs a steering wheel, an accelerator pedal and a brake pedal to go safely from point A to point B.  And if someone is texting and driving and veers into your lane you can do a few things to avoid a collision.  Such as stopping quickly by stepping on the brake pedal.  Twisting the wheel quickly to move out of the way.  Or stomping down on the accelerator to pull ahead quickly.   The combination of steering, brakes and accelerator can help us avoid many collisions.  Something a train can’t do.  Because a train doesn’t have a steering wheel.  And needs about a mile to stop.

This is why we stop for trains.  And trains don’t stop for us.  Because we can stop in a much shorter distance than it takes a train to stop.  Which is why trains have the right-of-way.  And we sit at railroad crossings.  Also, without a steering wheel they can’t steer around an oncoming train.  Or around a stopped train ahead of them.  The only thing a train can do to avoid colliding with another train is to stop before hitting one.  Or not being where another train is.  The preferred method of avoiding train collisions.

So how do they keep one train from not being where another train is.  Well, they’ve used many different methods over time.  One of the earliest methods was scheduling trains by a timetable.  Say there is a section of single track connecting two cities.  At, say, 8AM one passenger train leaves point A.  While another passenger train leaves point C.  They travel towards each other on a single track.  At approximately 9AM both trains arrive at point B.  The timetable will have one train pull into a siding and wait for the other train to pass by on the main line.  After the other train clears the track between point A and point B and continues on to point C the train on the siding will return to the main line and continue to point A.  According to the timetable.

Shorter Blocks mean less Waiting Time for a Train Ahead to Exit a Block

Of course, the timetable had its faults.  Such as when two trains were traveling in the same direction.  For example, let’s say train A and train B are moving from point C to point B to point A.  Train B leaves 2 hours after train A.  Which provides a two hour separation between trains.  Allowing train A to clear the track long before train B comes through.  Unless, of course, train A breaks down.  Which would be very bad for train B coming around a bend at speed only to see the rear end of the stopped train A.  And with no steering wheel or enough distance to stop train B would run into the back of train A.  Causing great damage.  And loss of life.

The timetable also made for inefficient use of track.  For it required large time separation of trains.  Which meant fewer trains in a given period of time.  And less revenue.  To increase revenue they had to shorten the time separation of trains.  Without decreasing train safety.  And the telegraph allowed us to do that.  With faster-than-train communication we could send new instructions ahead of a train (i.e., a train order).  Such as at the next station it will reach.  Telling them to stop on a siding for a priority train to pass.  Or to proceed slowly and be prepared to stop when they reach a broken down train ahead of them.  Etc.

We separate track into blocks.  For example, the portion of track between point A and point B is one block.  The portion between point B and point C is another block.  Trains travel through a series of blocks to get to their destination.  And to maintain the separation between trains they limit one train in a block at a time.  Ideally they want two empty blocks ahead of all trains.  So they can travel at speed through one block and have an empty block ahead of them for stopping room.  Areas with little traffic will have longer blocks than areas with more traffic.  For shorter blocks mean less waiting time for a train ahead to exit a block.

A Green Light means the next two Blocks ahead are Clear

Blocks started and ended at stations.  Signal towers.  Or block signals.  The last thing a crew does before moving their train from a stop is test the train-line air brakes.  The engineer will listen to the radio until he or she hears, “Got a good set and release.”  Meaning the brakes applied and released and were safe and functioning.  “Highball from the car department.  Have a good trip.”  The authorization to proceed.  The ‘highball’ is a reference to one of the first mechanical block signals.  A ball hoisted up by a rope and pulley.  The ball had three positions.  The high position meant the track was clear and the train could proceed at full speed.  The low position meant to stop.  And the middle position meant to proceed but to be prepared to stop at the next signal.

The semaphore was a common block signal before signal lights.  A semaphore was an arm that pivoted on one end.  When it was straight up it mean the track ahead was clear.  If it was at a 45-degree angle it meant proceed but be prepared to stop at the next block signal.  If it was horizontal it meant stop.  For there was a train in the block ahead.  Operators at signal towers would report when a train left its block to the signal tower at the entrance to that block.  So that signal operator could change the signal to clear.

Today we use electric signals.  And automation.  When a train enters a block its steel wheels and axles complete an electric circuit between the rails.  Turning the signal at the entrance to this block red.  There’s a variety of signal lights.  There are two-light units with a green light over a red light.  A green light means the block ahead is clear.  And the block ahead of that is clear, too.  Providing a 2-block separation between trains.  If the light is red it means there is a train in the block ahead.  And to stop.  If there is a green light over a red light it means the block ahead is clear but the next block is not.  So proceed at normal speed into the next block but be prepared to stop at the signal at the entrance of the following block.  Another style of signal light, the searchlight, had a single color lamp and three different lens colors that changed the color of the signal.  Green meant the block ahead was empty.  Yellow meant the block ahead was empty but the next block after that wasn’t.  And red meant to stop because there was a train in the block.  Perhaps the most common signal is a 3-lamp unit with green over yellow over red.  The signaling is similar to the 2-lamp unit.  But other combinations of colors provided additional information and direction.



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