HESITATION WHEN TRAINS LEAVE STATION ???

Can someone explain why, on occasion a train will start to depart the station only to quickly slow to a stop after about 10 yards? Sometimes this repeats 3 or 4 times before the train seems to gain momentum and start to accelerate normally.
Thanks in advance
Sam

I would imagine it is the slack being taken in or out when the train is departing.

Dave Becker

"snic2880"
Can someone explain why, on occasion a train will start to depart the station only to quickly slow to a stop after about 10 yards? Sometimes this repeats 3 or 4 times before the train seems to gain momentum and start to accelerate normally.

Loss of cab signal.

The gain on the track circuit amplifier that transmits the cab signals from wayside to the trains is not consistent over the length of the track circuit or form one track circuit to the next. The trains onboard receivers are not always able to hear of the signal when the signal strength drops below a given level. It's kin to driving under a bridge when listening to a distant AM radio station.

Several years ago WMATA was having problem with an inconsistent loss of cab signal in one of the Potomac River tunnels. Some trains would proceed through the tunnel with no problems others would lose cab signals and come to a stop. There was much puzzlement among WMATA engineers as to what was the culprit. The signal folks ended up increasing the gain on the amplifier in the track circuit where trains were losing cab signals. Problem solved.

Thanks Sand Box John, however, I am not clear what "cab signal" is. It would seem that as the operator increases the throttle to start away from a stop there is no "radio frequency" signal, but rather a direct connection (electrically) from the throttle to the motors in the cars. Am I simlyfing too much, is there more involved?
Thanks
Sam

"snic2880"
Thanks Sand Box John, however, I am not clear what "cab signal" is. It would seem that as the operator increases the throttle to start away from a stop there is no "radio frequency" signal, but rather a direct connection (electrically) from the throttle to the motors in the cars. Am I simlyfing too much, is there more involved?

Don’t confuse the connection of the operators controller handle to the propulsion system with 'Cab signals' or 'Speed Commands'.

The cab signals or speed commands tell the train how fast it may go.

There are 3 speed displays on the operators console. The left display shows Limited speed, the middle display shows Regulated speed, the right display shows actual speed of the train. The Limited and Regulated displays are what I am talking about when I use the term cab signals or speed commands.

If one or both of the Limited and Regulated displays (usually both) goes to zero, that is loss of cab signals.

When those displays go to zero in automatic mode the train will automatically apply the brakes until the train come to a complete stop or until the display start showing a speeds again . In manual mode an alarm will sound in the cab indicating to the operator to move the control handle to one of the brake positions. The operator has a limited period of time between the sounding of the alarm and moving the control handle to one of the brake positions before the trains fail safe system cuts in and fully stop the train.

The limited and regulated cab signal displays get their information from wayside by way of audio frequencies (8 channels ranging from 2,100 to 3,900 KH) transmitted through the running rail track circuits. Channels 1, 3, 5 and 7 are assigned to track 1, channels 2, 4, 6 and 8 are assigned to track 2.

There is an antenna array under the cab that receives the signal transmitted through the running rails.

Additionally there is 2 other channels (channels 9 and 10 in the 9,800 HZ range) used for train to wayside communication. Information transmitted to wayside from trains are train number, train length and destination.

Wow...I never realized how involved a system it is. Often the operator is announcing the the next station and is interrupted by a beeping just as the train stops accelerating, I suspect that is the loss of signal warning to apply the brakes.

I am new to Metro and for once in my life I am enjoying my commute! Thanks for the detailed explanation.

I want to say that's the overspeed alarm you're hearing, which goes off whenever the train operator exceeds the speed limit in manual mode.

"snic2880"
Wow...I never realized how involved a system it is. Often the operator is announcing the next station and is interrupted by a beeping just as the train stops accelerating, I suspect that is the loss of signal warning to apply the brakes.

The 'over speed' alarm as SchuminWeb properly identified will sound when the speed commands change to a lower speed then the actual speed of the train when being operated in manual mode. That is not loss of cab signals or speed commands it is just a reduction of the limited and or regulated speed commands that has changed as train moves from one track circuit to the next.

Brakes will only be applied by the fail safe system when the train is being operated in manual mode if the operator fails to apply them within a specified period time. I don't recall what that time is, but I think it's 5 seconds.

I am new to Metro and for once in my life I am enjoying my commute! Thanks for the detailed explanation.

Your welcome.