Another TPWS Question

[Peter]

I got this by e-mail

I have a query regarding TPWS as I read notes saying TPWS is still using across UK and it consists two parts. One is TSS and other is OSS.

Would you mind telling me more about the functions of the TSS and OSS?

What is difference of TPWS and ATP?

Have a look at this thread and in answer to the specific questions:

Yes it is in widespread use and has
- the TSS located at the signal which trips the train if it passes over the loops at any speed when the loops are energised.
- the OSS located a calculated distance before the signal which is energised with the signal at red and is designed to trip the train if it is going faster than the "set" speed which is the appropriate speed that a train is going to be able to stop at the signal.

TPWS only intervenes at given places if the train is going too fast or attempts to pass a signal at danger. Unlike ATP systems, it does not control the train in any other way. TPWS does not prevent SPADs, it mitigates the effect of them by making sure that the train comes to a stand within the "Safe Overrun Distance".

Peter

[PJW]

TPWS does not prevent SPADs, it mitigates the effect of them by making sure that the train comes to a stand within the "Safe Overrun Distance".

Perhaps better worded that TPWS is not INTENDED to prevent SPADs and the TSS certainly does not.
Depending on the train and the set speed and position of the OSS then because it does trigger an emergency brake application some distance prior to the signal, then it is ceertainly possible that the train may come to rest before it even gets to the signal- it is rare but there have been documented cases of this.
When they were first installed the bufferstop OSS freuently stopped a train from even reaching as far down the platform as it should- let alone the buffers.
I agree though that in the overall scheme of things, what Peter said was 99% true.

[Jerry1237]

There is an arguement that TPWS is a form of non-continuous ATP. However, it is not fail safe, i.e. it loses power, it will not trip the train.

Both the GWML and Chilterns use a form of non-continuous ATP but that system is fail safe and can be modified to improve the track-track reception frequency by adding additional loops etc.

Jerry

[PJW]

There is an arguement that TPWS is a form of non-continuous ATP. However, it is not fail safe, i.e. it loses power, it will not trip the train.

Both the GWML and Chilterns use a form of non-continuous ATP but that system is fail safe and can be modified to improve the track-track reception frequency by adding additional loops etc.

Jerry

True but if a loop loses power then the proving will be lost and the signal in rear will go to red and therefore the TPWS should intervene there. I agree it is no failsafe, particularly on board and that it is a bit of a misnomer- however it is infinitely better than not having anything at all.

[khlai]

Dear Peter,

What are the legend of TSS and OSS?

Should the field equipment need to be shown in the layout as an answer to the module 2 of IRSE examination?

Regards,

Henry

[PJW]

Dear Peter,

What are the legend of TSS and OSS?

Should the field equipment need to be shown in the layout as an answer to the module 2 of IRSE examination?

Regards,

Henry

TSS = Train Stop System
OSS = Over Speed Sensor (but to be honest sensor is not what I'd call it)
TSS at the signal, set to intervene whatever the train speed
OSS on the approach to signal or bufferstop with a set speed determined by the spacing of the loops and acts as a speed trap for a train crossing the loops above that speed if loops are active.

I would show for bay platforms approaching buffers (because most layouts would only have one or two) but certainly not waste time showing on every signal. Cover by general note as advised in Study Pack and see some posts in Module 2 layouts section where students have suggested some phrases that they intend to utilise