Operating Parameters

Railways not only have safety rules but also policies which determine how they operate. Some of these are judgmental, in that (for instance) tighter specifications may be possible but may lead to reduced punctuality, or reduced safety.

Ruling line speed: a railway is designed for a particular ruling line speed, at which speed any cant deficiency will be comfortable for passengers. This will be the basis on which trains are timed, unless there are other limiting criteria (e.g. adverse gradients). The speed will be affected by the type of traffic; as the NRNE project is primarily freight‐driven, ruling line speeds do not need to be particularly high, and a value of around 110kph may be sufficient. If it becomes important to improve inter‐city passenger services, then higher speeds up to 200kph may be adopted in future. This corridor has a requirement for a high ruling line speed (approx 150kph) in the key section in the Rift Valley between Mojo and Shashemene, with lower speeds on either side of this; to the North, the many local station stops mean that a higher speed is not necessary, whilst to the South, the landscape is more mountainous, demand is inherently lower and hence the expense of higher speeds unjustified. Ruling line speeds of 90kph should be sufficient to enable train services to average 50‐70 kph.

When trains are timed, allowances need to be made for such factors as:

• recovery from temporary speed restrictions;

• recovery from delays caused by passengers at stations;

• variability between trains and drivers, etc.

Some railways use a percentage method for calculating recovery time, in that a given small percentage (of the order of 5%) is added to the minimum times quoted by (for instance) locomotive manufacturers. Other railways add a number of minutes (e.g. 5) before major junction stations. Whatever method is chosen, it should be noted that this needs to be re‐visited once operations begin. However, what is more critical is that such allowances are included, otherwise performance will deteriorate unnecessarily.

Once a railway is in operation, trains will inevitably occasionally run outside their planned timetabled arrangements, and decisions have to be made in the control centre about train prioritisation. Although it may appear fair to operate on a ‘first come, first served’ basis, or to give trains which are on time precedence over those which are not, neither of these policies is likely to minimise delays. Trains with longer distances to travel to their destination have a greater likelihood of causing knock‐on delays to other services, and so should be weighted favourably. Whether the control centre is operated on a day‐to‐day basis by signal staff or computer program, rules need to be set out in advance.

When trains pass at junctions or (on single lines) at passing loops, consideration needs to be taken of the facts that trains are relatively slow to brake and accelerate, and the setting‐up of the next signalled route is not instantaneous. It may theoretically be possible for one train to leave a passing loop one second after the opposing one has entered it, but this will lead to unnecessarily‐poor punctuality. It is sensible to allow a small number of minutes for such movements, if delays are not to propagate unnecessarily.

Similarly‐detailed procedures need to be thought through for the entire operation of the railway, and set down in the rule book. A specific concern for NRNE, given the importance of the traffic concerned, are operational procedures for loading and unloading oil traffic; this might require locomotives to be switched off, or even that wagons were uncoupled – although the latter should be avoided, as it could significantly increase shunting times.

Дата добавления: 2015-07-20