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No, the constraint is the fact that all except a few trunk freight lines are designed for relatively infrequent and long freight trains, with a single line with "passing loops" (this may be an aussie phrase) or sidings that the train going one way sits on until the train going the other way has passed.

Since the freight has priority, once a passenger train in the US leaves the passenger only lines in the Northeast, small delays cascade into massive delays as it sits in a passing loop, waiting for train A to go past a passing loop further up the route so that train B can get off the passing loop and go by the Amtrak so that the Amtrak can move. A delay of 15 minutes can easily spill over into a delay of three to five hours in tack access delays.

Now, most of these are in rail corridors that once had two way track (because there used to be far higher frequency of passenger trains and much more local freight travelling by rail). So in these corridors, you can take advantage of the existing rail right of way to  put in a dedicated passenger service track.

If the frequency is brought up to the level where passing loops are needed at all, the passenger line can take advantage of the long stretches between freight trains by having switches that allow either the existing freight track or existing freight passing loop to be used as a passing loop for the passenger services.


I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.

by BruceMcF (agila61 at netscape dot net) on Thu Jan 25th, 2007 at 10:53:30 PM EST
[ Parent ]
Ah, so for your MHS, you were thinking in train frequencies like Amtrak long-distance?

I think if one bothers about speed at all, then only when passengers can  rely on frequent trains. I mean, it makes little difference if a travel takes 3 or 5 hours when you would leave at 8 o'clock but the only trains are at 6 and 18 o'clock.

Regarding two single track, use the other for passing vs. one double-track operation, let me demonstrate that the difference is significant with the following virtual train scheme I generated.

This is a 50-mile line between two major cities, with 12 sections. The borders of the sections are considered both crossover points and stations for local trains. I put paths of twice-hourly passenger trains with 50 mph travel speed and once-hourly medium-high-speed trains with 133.3 mph top speed on it in both directions. (For simplicity, different gradient lines represent acceleration/braking for the fast trains, I didn't bother to resolve that for the local trains.)

The second line for each path represents a 2.5-minute buffer for lateness. Then: the orange fields show time-distance zones when trains can pass each other, while the grey zones represent a train travelling 'on the wrong track'. In the first mode of operation, all the grey squares must be kept free, and there is room left for only one long non-stop freight train with 50 mph per direction. In a double-track operation, you only have to watch out for the fast trains and their crossing of same-directional trains, room for around four freight train paths in both directions.

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Sat Jan 27th, 2007 at 06:18:27 PM EST
[ Parent ]

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