Truckers Like This..

[PepperElf]

Well with trains I can see that... Even at 5mph the mass of any train can do some damage. But on top of that, trains don't have to worry so much about whether or not their speed - or lack of it - impedes other traffic.

[dalesys]

On this side of the pond...

In Yourp a few hours at 90¹ kph is measured in countries. Here at 90² mph it's in counties!

¹90 kph ≈ 55 mph
²90 mph ≈ 145 kph

[Geek King]

As for speed, I've never heard of a national limit.

There was, at one point, a national speed limit of 55mph, later raised to 65mph, then done away with altogether on a national level. I don't think it varied for big rigs and heavy vehicles on the national level, though.

[Chromatix]

But on top of that, trains don't have to worry so much about whether or not their speed - or lack of it - impedes other traffic.

Actually, they do. Not so much the drivers, but the planners certainly.

An example I used elsewhere was to consider a two-track line (one track per direction) 20 miles long, say between two sections of four-track line. Passenger trains can (in theory) follow each other at 2-minute intervals at 120mph. Freight trains can follow each other at 5-minute intervals at 60mph. This is normal spec if the signals are about a mile apart.

Now consider if you mix passenger and freight traffic on that line. Say a passenger train goes first, immediately followed by a freight train (starting 2 minutes later). The passenger train will arrive after 10 minutes, the freight train 12 minutes later than that. The next train can't reasonably arrive until 5 minutes after *that*, so if it's a passenger train, it should aim to start a whole 17 minutes after the first passenger train left, in order to avoid being held up.

But you could have fitted 7 more passenger trains in between these two, if the freight train were not there. Railway planners therefore say that the freight train "consumes seven passenger paths".

A second freight train can of course start on minute 19, 17 minutes after the first freight, and will not be held up. But another 2 freight trains could have been fitted between them if the second passenger train were not there. So the passenger train, likewise, "consumes two freight paths".

Overall, the efficiency of a track with different speed trains on it is much lower than one where all the trains run at about the same speed. That's why railways are so often built with four tracks instead of two, so that fast traffic can have a line to itself, and so can slow traffic.

[PepperElf]

o yeah, i can see planning will take some serious thought. that's why trainwrecks are so horrendous.

if anything though the masters at planning are the japanese. you really can set your clock by the trains in japan.

but mainly i was thinking trains vs cars


actually my bf and i agreed on one thing... that it would be cool to have a train built in the middle of the local highway if it could be done.

[Andara Bledin]

Do you mean like how the Green Line in LA runs down the center of the 105 freeway?

^-.-^

[SuperDan]

actually my bf and i agreed on one thing... that it would be cool to have a train built in the middle of the local highway if it could be done.

The Chicago Transit Authority's red and blue lines do run down the center of Interstates 90, 94, and 290. The tracks run right down the center between the two directions of the highway, with stations situated between the two tracks and accessible via stairways from the streets that pass over the highway. Here is a map showing one of the stations.

There is a stretch of city street in a city east of me where the local commuter line's tracks go right down the middle of the street, no partitions or anything. Through this particular stretch, the trains obey the same 30 mph speed limit that cars do and give a very short blast of their horns every ten seconds. I don't remember offhand if the trains also have to stop for traffic lights or if the lights are on sensors to change automatically for the trains. I panned up and down the line in Google satalite view and didn't catch an image of the train on the line, but here is a Google Maps view of the station that is in this stretch of track. The evenly spaced utility poles matched on either side of the road actually hold the overhead power lines for the electric trains.

[Shalom]

For similar reasons, freight trains tend to have lower speed limits than passenger trains over here. Apparently in America, it is just as often the other way around.

Generally, the speed limits depend on the class of track, but the limits for freight in USA law are lower than passenger in all classes up to Class 5 (80mph freight, 90mph passenger). Once you get into high speed rail territory (Class 6 and above), the limits are only defined for passenger service, so it's possible freight isn't even allowed on those lines. Not sure about this, but there's hardly any Class 6 in service, and most of it is in Amtrak NEC territory which doesn't see much freight anyway.

Besides that, there's a hard limit of 49 for freight and 59 for passenger on unsignalled tracks, regardless of their class, and 79 for anything not equipped with cab signals or automatic train stop. Since most lines don't have ATS or cab signalling implemented, most passenger trains operate at 79 or less.

I read somewhere that most general freight tends to move at around 45mph even if they legally can go faster, as the railroads found that this is the best trade-off between fuel economy and speed. Hot intermodal trains (UPS, etc.) where speed of delivery is essential tend to go faster than this; bulk commodity trains (coal, grain) where you order it a month in advance and don't much care which particular carload you get, go slower.

In Yourp a few hours at 90¹ kph is measured in countries. Here at 90² mph it's in counties!

The difference between Americans and Europeans:

Americans think 100 years is a long time.
Europeans think 100 miles is a long distance.

Overall, the efficiency of a track with different speed trains on it is much lower than one where all the trains run at about the same speed. That's why railways are so often built with four tracks instead of two, so that fast traffic can have a line to itself, and so can slow traffic.

The Pennsylvania RR did have a four track mainline for this reason, as did the NY Central.

But unless the traffic is really heavy, I'd think three tracks might be enough, with enough crossover points that trains can go around other trains without encountering oncoming traffic. You might even get away with only two tracks, if both are signalled in both directions, there are enough crossovers, and the traffic isn't quite that heavy.

[Chromatix]

Then that's yet another difference from European practice - if we have a two-track line, one track is for each direction. Running on the wrong-direction track is called "bang road" - have a guess why - and is for exceptional occasions only, such as the usual track being blocked. Track with multiple bidirectional lines does exist, but for short distances in complex junction areas.

Part of this is because using a crossover requires slowing down. It's not at all unusual to have a 90mph mainline with 30mph crossovers, or a 125mph line with 70mph crossovers.

For this reason, three-track lines are very rare here. There is one about 20-30 miles away from me, which is effectively a two-track mainline with an adjacent single commuter line, simply to get the relatively sparse outer-suburban traffic out of the way of the fast mainline traffic. In theory it could also make sense to have a slow line on the uphill side of a steep bank but not on the downhill side, but unlike the road system I don't know of any places where there is one.