Good engineering beats unobtainium

A few years back, when the first Siemens Desiro EMUs were entering service, I introduced a new technical term to the rolling stock lexicon: lardbutt.

Compared with the previous generation of EMUs, the Desiros were heavy. Add in stiff Continental bogies and they proved to be track crunchers, eventually requiring an innovative variable rate damper modification to reduce track wear.

When the new trains for Thameslink and Crossrail went out to tender the need for minimal weight was a feature of the invitations to tender. After the contracts were let the successful bidders assured me the lardbutt era had ended. Well they would say that, wouldn’t they? First of the new trains in the metal was the Class 700 Siemens Desiro City for Thameslink. So when Siemens invited the press to see the trains at its new depot, I spent some time walking down a 12-car unit noting down the painted numbers on the vehicle ends.

A subsequent trip on a Bombardier Class 345 Aventra proved fruitless in terms of weight research as the vehicle ends house cables, leaving no room for painted weights. However, a quick call to Derby provided the necessary information.


When I put the numbers into a spreadsheet it was clear both manufacturers had delivered on their promise. But weight is relative and we need a sense check on previous train weights. Comparison is not easy because the longer a train, the more vehicles you have to spread the equipment over.

There are not many eight-car or longer EMUs against which to compare these new generation trains. The best I could come up with was the now defunct eight-car Alstom Class 460 built for Gatwick Express.

Bear in mind this is 750V DC only and so does not have to carry transformers and pantographs around, as do the Bombardier and Siemens lightweights.

With five vehicles motored, the average vehicle weight is a shade under 40 tonnes.

After that introduction we can look at the table. And I find the new trains truly impressive.

Obviously the 12-car Class 700 average benefits from those very light trailer cars, but to get the average vehicle weight to 33.53 tonnes is a considerable achievement.

This is the same as a standard Mk 3 coach, which has long been my gold standard for efficient vehicle design.

When the Mk 3 was introduced over 40 years ago it was the cheapest and lightest air-conditioned loco-hauled passenger vehicle in Europe. In addition to lightness it was proved to have excellent crashworthiness and the body shell was very stiff, allowing the BT10 bogie to work in ideal conditions for that smooth ride.

Crossrail unit: EMU No 345006 leaves Stratford en route to Liverpool Street with a service from Shenfield on 5 February 2018. Chris Playfair

At first sight, the nine-car Aventra, with an average of 35.4 tonnes per vehicle, seems less impressive even compared with the eight-car Class 700 at an average of 34.4 tonnes.

However, the Class 345 is formed from 22.5-metre vehicles compared with the 20.2 metres of the Class 700, so you are getting more floor space for that extra tonne per vehicle.

But this is mere sophistry. The reality is that both Bombardier and Siemens have produced two very impressive lightweight designs, which are going to produce substantial benefits throughout their working lives.


Obviously, a lighter train uses less energy to accelerate and with all those motored axles you should recover at least 20% through regenerative braking.

But, note the bottom lines I have added to the tables.

Train operators pay a Variable Usage Charge (VUC) to run their trains on Network Rail infrastructure.

The VUC is intended to reflect the cost of the wear and tear caused.

The calculation takes into account axle load, bogie stiffness and various suspension parameters.

In the table I have calculated the VUC in pence per mile for each train. Once again there is not a lot in it, although the nine-car Class 345 costs around the same per mile as the eight-car Class 700. However, both the new trains cost a third less per mile than the Class 460.

Cost per mile is not a readily accessible parameter, so I have made some calculations based on Great Northern fleet mileage.

Running the current Class 700 diagrams with eight and 12-car Class 387 formations would, I estimate, increase GTR’s annual VUC from £5.5 million to just under £9 million.

Obviously, this is a rough approximation, but it does indicate the practical benefits of lightweight trains which accumulate over their service life. Congratulations all round. And the next time someone blathers on about the need for innovative trains built with lightweight materials… they can now expect a numerical duffing up.

Comparator: the Class 460. This is No 460004 at Sevenoaks on 11 June 2007 en route from Stewarts Lane to Chart Leacon for a Level 6 overhaul. Brian Morrison
Thameslink train: unit No 700130 at South Croydon with the 14.12 London Bridge to Brighton service on 25 April 2018. David Andrews