Why do trains sometimes slow down on some sections instead of continuing at the same speed throughout?
There are many reasons for a reduction in speed. There may be permanent speed restrictions on the section of track: because of sharp curves or curves with inadequate cant; approaches to crossovers, diamonds, etc.; structures too close to the track; ghat sections; lineside tenements or pedestrian traffic; level crossings; old bridges or culverts; inferior track or lighter rails than normally required; unstable trackbed; frequent threat of flooding, etc. The working timetable usually has a detailed list of these restrictions for all sections within a division.
There may also be temporary speed restrictions (also simply temporary restrictions) such as engineering speed restrictions because of construction work or track maintenance, or because of flooding or other track damage, etc., all of which necessitate following the appropriate caution orders or caution notices in force for the section. See below for more on this. Sometimes newly-laid track may not yet have been certified for higher speeds while lower speed traffic is allowed.
A Caution Order (or caution notice) is a written notice issued by a station master (or other official) to the driver and guard of a train, formally advising them of special conditions and restrictions in effect on the section of track that the train is about to enter. The Caution Order may have instructions on speed restrictions and other special procedures to be followed on account of damage to the tracks, flooding, work on the permanent way or on the electrical equipment, accidents (or reminders of spots where accidents recently occurred), work on or damage to OHE equipment, or unusual situations.
A caution order can also be issued to advise the driver and guard of the presence of manually operated or motor trolleys, tower cars, MOW wagons, or other such maintenance or emergency vehicles that have entered the block section ahead. The caution order usually specifies the location of the affected section of track, the temporary speed limits in effect, the locations of caution indicators and termination
Some representative examples of caution orders are the following:
- Track doubling in progress - whistle to alert men at work
- Track destressing - 20km/h
- New colour-light signal location
- Level crossing gate - no acknowledgement given; be prepared to stop if gateman does not display hand signal
- Accident spot - 75km/h
- Up distant signal number ... of station ... inoperative due to a cable break; keep a good look-out, whistle while approaching and Proceed
A caution order is generally issued by the station master of of a station adjacent to the block section which is affected. In addition, divisional caution orders are also issued by station masters of certain specified stations on the route, known as notice stations.
A caution order is specifically addressed to the driver and guard of a particular train identified on it. Separate caution orders are issued for each train passing through on to the affected section. At many of the larger stations nowadays the caution orders are printed out but at smaller stations, handwritten notes still
A nil caution order is issued by a notice station to inform the driver and guard of a train that there are no special caution instructions or temporary speed restrictions in effect between that station and the next notice station. A reminder caution order may be issued by a notice station to reiterate caution orders already issued
by other stations or authorities.
SPEED RESTRICTIONS AND ENGINEERING RESTRICTIONS
These are various kinds of speed limits below the normal sanctioned speed limit for the route section in question, imposed in stretches of track where unsafe conditions exist because of track damage, ongoing repair work to track or OHE, accidents, or unusual circumstances in the construction of the permanent way
A temporary engineering restriction is specifically one that is imposed for a fixed duration on account of ongoing work on the permanent way or OHE equipment; a permanent engineering restriction is one that is in effect indefinitely because of characteristics of the permanent way. Other temporary speed restrictions may be imposed because of flooding, track damage, accidents, etc. A stop dead restriction is one which requires a train to come to a complete halt before obtaining permission to proceed.
For short-duration (1 day or less) temporary speed restrictions, hand signals are used at appropriate points (30m to the rear, and 800m (more in some cases) to the rear) to advise drivers of the location of the restriction. For a short-duration stop dead restrictions, a red banner flag is placed across the tracks just before the obstruction, and another banner flag is placed beside the tracks 600m (BG; 400m for MG/NG) before the location of the affected portion of track. Three detonators are also placed 10m apart, about 1200m (BG; 800m for MG/NG) before the banner. Hand signals are used 30m to the rear of the obstruction and 45m to the rear of the detonators.
For longer temporary speed restrictions (lasting more than a day) a speed indicator is placed 30m to the rear of the affected portion, and a caution indicator 800m (or more in some cases) to the rear. For longer stop-dead restrictions, the speed indicator is replaced by a stop indicator, whereas the caution indicator is placed
1200m to the rear (BG; 800m for MG/NG).
Normally the driver and guard of a train are issued caution orders that provide details of the temporary restrictions. Caution orders are not issued for permanent restrictions of any kind.
The caution or stop indicators, banner flags, hand signals, and speed indicators are all dispensed with if the affected portion of track is within station limits and if it can be appropriately isolated by the settings of points and leaving signals protecting it 'on' (at danger). The caution indicator is also dispensed with if the affected portion of track is protected by an automatic signal less than 1200m (BG; 800m MG/NG) from the obstruction. In this case the detonators and banner flags are placed at 180m and 90m to the rear of the obstruction for a stop-dead restriction