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catalanmercenaries @aussie.zone

Why can't light rail use the same rollingstock and tracks as heavy rail?

i've always wondered this

lightrail, streetcars, trams. Why must they be built to be incompatible with heavy rail? why can't heavy rail be built with a bunch of level crossings, street level stations, slow speeds, and function exactly the same as light rail?

if compatible, the rail could act heavy at designated tracks and light at others, removing what would otherwise be an interchange. it would also allow the light rail to have a higher top speed at express areas.

what am i missing?

6 comments

  • Also in some countries, cars ride on the right but heavy rail ride on the left (because Britain).

    Light rail need to drive on the same side as other surface transport because they share road. Else they need to be fully separated, which is costly enough where at this point it is better to build heavy rail.

    So you will always have a complex interchange between light rail and heavy rail on those countries.

    Not to say it doesn't happen but it does hamper this concept.

  • Light rail is typically used in situations where it is fitting the trains into smaller spaces, often sharing space with other modes of transport. Building it to a heavier grade would cost more and offer no benefit. Even if you spend the money to build a light rail line to support a longer, heavier, faster train, you could not run that train anyway, because the spaces is shared, the turns may be too tight, the bridges may be too low, there may be no space for platforms. It is more cost and space effective to have separate lines.

    Note, many light rail networks use standard gauges, so they can use standard maintenance equipment.

  • Basically because it's light.

    For example in Karlsruhe, Germany, they do have a system where you enter what is essentially a metro station (a.k.a. "a subway station"). You then enter a train that leaves the tunnel after a couple of stations and starts turning in the intersections above the ground and stopping in traffic lights. After passing the central railway station, it enters the railway tracks and proceeds to run the next 40 minutes along the very same railway tracks used by cargo trains and high speed trains.

    So yeah, that can be done and is being done. But it is a very complex thing to do!
    By far the most important difficulty is that the tram is too light to be safe on the railway tracks just like that. If there is a collision between a train and a tram, the heavy train will absolutely obliterate the tram. There won't be any survivors in the tram if that ever happens. This means, you need to clear a very long stretch of the track of any heavy trains whenever a tram is about to use it. This decreases the capacity. A tram also cannot be reasonably made to run 160 km/h like passenger trains usually do, and speed differences between trains are a horror for the railway's capacity.

    At the same time, the tram's ground clearance must be higher than usual for trams, which requires higher platforms, which can be cumbersome to organize in city streets. It also easily means that handicapped people have it more difficult entering the tram.

    If you want to make the trams crashworthy enough to have a fighting chance against a train in a collision, you will end up having such heavy trams that they cannot be used on city streets.

    There are situations where it really does make sense to run trams on railway tracks, but in most cases it's better to choose one or another instead of trying to combine a tram and a train into one thing. But, Karlsruhe has a good example of when this combination does make sense!

    Here's a Youtube video of a ride on a metro->tram->train vehicle :)
    A high-speed train is encountered at 35:26 and a cargo train at 38:48.

  • From my North American engineering perspective:

    The main thing is that when you look at a new transit project in isolation, the cheapest thing to design and build is a transit system that interacts with as few existing parts as possible. There are plenty of exceptions to this, such as using an existing rail tunnel through a mountain vs. digging a new one, or if an existing maintenance facility has enough spare capacity to avoid needing to build a new one. But in every place the new line interacts with existing infrastructure, it's a cost to determine how the design can best integrate it both during and after construction. Take an interchange station with another line. Will it go beside, above, underneath? Can it be made without needing the close the station or the entire line, or with as little disruption to existing service as possible? Interchange stations are great for usability, but planners and design bidders evaluate and present cost-to-benefit tradeoff scenarios that will get approved or denied for both fiscal and political reasons. Let's say between two distinct lines you want to have shared track. If the electrification type, rail gauge, signalling system, platform length, vehicle profile, boarding level height are different, you will have to spend a solid amount of engineering effort figuring out how to harmonize it.

    There are many examples of light rail vehicles using track or right-of-way shared with heavy rail in full or partial sections of the line. Waterloo, Canada's ION tram is one. Unfortunately it's much more difficult and costly to have it the other way around-- i.e. heavy rail vehicles on light rail tracks. The tracks and roadbed are not meant to handle the weight and vehicles may not make it through the smaller bridges tunnels, and curves. To make a light rail system compatible without knowing what heavy rail trains would use it, is a major cost incurred for no forseeable benefit.

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