Liverpool Railway News and Discussion

[Martin S]

With the right rolling stock a newly morphed metro can then extend in to Wirral and Liverpool Waters. DLR type of roling stock is the first and vital move for Merseyrail and the city as a whole.

This is a more technical issue and I have given this one some thought as, on first sight at least, there is some merit to it.

It is often surprising how trains that perform relatively similar functions are so different. So, for example, DLR trains are very different from London Underground tube stock, Manchester Metrolink trams are very different from Croydon trams and Merseyrail trains are different from London Underground sub-surface stock.

Some of it is obviously down to styling and the structure gauges (nobody is going to recommend Class 507s for the Glasgow Subway without a fair amount of 'jiggery-pokery'). However there are some more subtle issues.

If you want to decide on what the best rolling stock for a particular rail system is, you need to have some idea of what the purpose of that rail system is. Merseyrail has some 70 route miles of mainly suburban and outer suburban railway of which only 6.5 miles could be described as an underground metro system. Consequently, you are going to select rolling stock that is best suited to suburban type operations.

That involves compromises. The Class 507/508 stock is not best suited for running in the tight radius Merseyrail underground tunnels - you only have to consider the problems there have been over the years with replacing rails and wheelsets due to excessive wear. That is mainly because the rigid wheelbase of the train bogies prevents the wheels from following the curve of the rails - they will always be at a small angle, which means that the wheel is trying to climb over the railhead. In extreme circumstances, that can lead to derailment, more often it just means more frequent replacement of wheels and rails.

One solution to that problem is the use of rail flange greasers - but these require regular maintenance and replenishment - the long term solution is the use of trains with special suspension systems that allow the wheels to follow the curves more accurately (that was the initial intention when the tunnels were built but, for some reason, was not implemented). Hopefully, when the rolling stock is replaced in 2013 or whenever, that issue will be resolved.

Use of DLR trains would not necessarily resolve the problem. As far as I am aware, they do not have the radial wheelsets that would be required. They are articulated - but that is to resolve a completely different problem.

DLR was initially conceived as an above ground railway, mainly on viaducts. In more recent years it has evolved into a partially underground system but the trains have remained largely the same.

Articulation of the vehicle bodies was required to enable trains to negotiate tight curves. Reducing the rigid length of the vehicle (14m as opposed to the 20m of a Merseyrail unit) means that the 'centre and end throws' (the overhangs formed by a straight vehicle trying to go round a curve) are reduced and the train does not have so wide a 'swept envelope' - an important consideration when trains need to go round buildings and for that reason trams are often articulated.

However, articulation doesn't exempt you from the laws of physics. If you go round a bend at speed, you will experience a high centrifugal force, which is uncomfortable for passengers and can derail the train, so for that reason DLR goes at much lower speeds than Merseyrail.

Using DLR vehicles on Merseyrail would probably cause far more problems than it would solve. Let's forget trivial details such as the different current collection methods, the different platform stepping distances and the fact that DLR are automatic trains (which would require complete resignalling of Merseyrail if it were allowed at all).

DLR units are primarily metro trains. They are designed to carry large numbers of passengers over relatively short distances. A Merseyrail train of comparable length to a DLR train would have some 50% more seats. That is not just the seating configuration, it is the fact that the individual vehicles are much shorter and so much space in the train length is given over to the connections between vehicles, more frequent and wider doors and the articulation. Also, the trains are significantly narrower than Class 507/508s.

John has suggested that the use of tight curving DLR trains would make his scheme for a metro system for Liverpool easier to achieve. That would be true if he was planning a system that was mainly above ground and had to snake around buildings - but most of his proposed system would be underground - and underground you can make curve radii as large as you like. In fact, if you think about it, a tunnel linking two straight lines at an angle would be shorter if the radius was increased, so construction costs would be lower and trains could run at higher speeds. The complexity of the junction (i.e. whether you would need a flat or grade separated arrangement) would not be affected.

So, using DLR trains would not be the answer but there may well be a case for using lighter units than we have at present. One reason that the suburban type trains used on our system are so heavy is that they are designed to protect passengers in the event of a collision with other heavy rail trains. With the system almost fully segregated from the main rail network, lighter rail units might be acceptable (although the speed of running must also be a factor). Lighter vehicles not only need less energy to accelerate or climb gradients but reduce damage to the track. I suppose that the aluminium bodied 507/508s are on the lighter side of 'heavy rail' but there should be scope for improvement.

Of course, when it comes to rolling stock replacement, it will always be cheaper to buy vehicles of similar specification to ones used elsewhere (due to the reduced cost of a long production run), rather than order bespoke vehicles specifically tailored to Merseyrail. It all boils down to economics in the end.