Theo_Fidel wrote:Arshyam,
I have one question for you.
Why should two trains on the same track have different speeds and priorities?
Theo sir, simple answer is: it is a function of the type of train on the tracks.
The detail is somewhat long, so please bear with me.
IR runs different types of trains with a varied rake mix:
Rajdhani: these are fully AC trains, so their max permissible speed (MPS) is 160 kmph.
Expresses: these are mixed coach rakes (AC+non AC), so their MPS is only 110. With new coaches, this can go up to 130. They are still limited by wind drag in non-AC coaches. Within this category, there are the following:
- Long distance super fast express trains (granted, they are not that fast by world standards) which focus on the end to end traffic. Consequently, they have less stops, and can be thought of a mixed rake Rajdhani. For example, TN exp, AP exp, etc. MPS:110.
- Long distance super fast express trains, which also focus on intermediate traffic. Consequently, they have more halts on the way. For example, GT exp, Howrah mail. MPS:110.
- Long distance regular express trains: These are not super fast, so charge a slightly lesser fare, and also stop at a lot of intermediate stations. For example: Andaman exp, East Coast Exp. MPS:110.
- Local passenger trains: These sorely serve the social mandate of IR and provide rural connectivity, so stop at almost all wayside stations. MPS:110.
- Suburban EMU trains: They serve the periphery of cities, and usually stop at all suburban stations (except fast services). MPS:80-100.
Goods: Depending on the type of cargo, their MPS can vary. An iron ore rake will run slower than a parcel cargo train, simply because of the effort taken for braking from a higher speed will be more for a heavier ore train. Also, heavier trains need multiple locos, and in case of electrics, our system is set up for only 110kmph max with multiple locos, due to oscillation issues in the overhead catenary equipment. MPS: 70-110.
Note: MPS is limited by track conditions as well, so usually is the lowest of loco MPS/rake MPS/track MPS. I have called out only the rake MPS above.
Some factors affecting speed and priority:
Time/Halts:
Now the MPS for a train is one thing, but can all trains run at their max speed? If they don't have many stops, sure, it is easy to maintain speed at a constant level. In fact, from an energy consumption PoV, it is preferable. So, Rajdhanis and the category 1 of expresses usually run at their MPS, and don't block trains behind them. Categories 2-4 can also run at MPS, subject to their halts. If their halts are too close by, then they will accelerate to a lower speed before arriving at the next halt. And each halt means time to slow down + accelerate. And chances are, these trains have enough stops that invariably a faster train shows up from behind, so the latter gets priority and overtakes the slower train. An exception are the suburban trains, with their lower MPS. They almost always get higher priority over any other train, due to commuter pressure. Within cities, that is another headache for the section controller.
Energy:
With local passenger trains, the halts might be so innumerable and close to each other that it is not efficient to go all the way up to 110. For goods, that is the same: a heavier rake will need time to accelerate and brake, and will require a lot of energy to do both. Plus, as I had said above, they are also limited by the number locos dedicated to the rake. So, it may just be efficient to run at an easier balancing speed for the loco, usually 70-80 and try to keep the speed as long as possible.
Fare: A premium train such as Rajdhani charges are higher fare, so the customer can expect a higher level of service. This is true for catering as well as scheduling. Since a passenger has forked over extra cash, he gets priority treatment in case of delays or congestion. Hence, Rajdhanis get to overtake any train on their path when out of schedule. Superfast express trains charges a surcharge over ordinary fares, the idea being that this surcharge gets priority treatment compared to a regular express. So the express train pecking order goes in the order listed, TN gets to overtake GT, the latter overtakes Andaman (non super fast), Andaman might over take some passenger train, and so on. Goods trains also have a pecking order, and they are actually interleaved with passenger trains. There are dedicated ConRaj trains (container Rajdhani) that are higher fare high priority trains that get right of way over most passenger trains. They run at 100-110.
So the answer is a mix of the above: depending on the train's clientele, the halt pattern is decided, and schedule is drawn up. Depending on accommodation, the rake is allocated, which has an assigned MPS. Depending on fare class, the priorities are assigned. In the end, the controller takes all of these into account and schedules trains in his section.
The above is not written in stone, but since we are limited by the space on tracks and a huge number of trains everyday, these issues crop up. A lot of the above can indeed be simplified by building more tracks, newer rolling stock and improving infrastructure overall.
This was an involved reply, but I hope it answered your query.
(sarc on)