Class 93: the science of modern traction 

class-93-locomotive (Stadler)

Rail Operations Group has not ordered a fleet of class 93 tri-mode locomotives just because of the funky livery. The Stadler-built locomotives, due for delivery from 2023, will not only help the industry address drive towards a net-zero carbon economy, but they will also help the company find new markets.

Karl Watts, the chief executive officer of Rail Operations (UK) Limited, Rail Operations Group (ROG)’s parent company, says that his company has ordered the advanced class 93 design to underpin new high-speed freight and logistics services. The fleet, eventually 30 in number, will according to ROG provide lower operating costs and greater operational flexibility. Doubtless that’s good news for the new owners, who recently acquired the company, but it’s good news for the environment too.

Brentry era traction alongside Brexit-era technology

The class 93 represents a development of the Stadler design already at work in the UK. The platform has been the basis of the successful class 88 bi-mode variant, which has proved extremely useful for other operators, particularly on diversionary workings, where normally electric-hauled flows have been required to divert away from overhead supply and revert to diesel power. The class 93 not only has a highly efficient diesel unit onboard, but it also carries a traction battery pack, which is available to power the locomotive, or supplement the diesel power unit.

The personable Karl Watts of Rail Operations (UK) Limited, is conversant with the technology of modern traction. Image: RO (UK) Ltd

There are motive power units operating daily on the UK network, some of which pre-date ‘Brentry’, and will soon be running alongside advanced designs like this class 93, which were hardly conceived in time for Brexit. “It is quite remarkable how much traction technology has developed over the past two or three decades”, says Karl Watts. The locomotive, he says, is just one of various innovations which his company is developing.

Tractive effort science in action

The industry standard class 66 diesel leaves everything else standing for tractive effort, the ability to start a train from rest. However, even though the ’93 has just four axles, and the ’66 has six, once the train is above walking pace, the ’93 keeps on pulling. “The control of tractive effort has improved so dramatically as to radically change the way locomotives behave on the track”, says Watts. “The older technology – thyristor control – is superseded by systems that deliver constant manipulation. At one time, wheel slip control that activated within an eighth of a wheel revolution would be industry leading performance. Now, the constant control available and systems built in to the class 93 is almost instantaneous. We have a spectacularly advanced traction control system on this locomotive.

Green ambition

Remarkably, individual axle control can be used to proactively enhance tractive effort. Wheel slip creep is designed to condition the railhead. Preventing the rail equivalent of burning rubber is critical, if damage to the railhead is to be avoided. “Allowing controlled slippage of the first axle, effectively conditions the railhead, affording better traction for the following three axles on the locomotive”, says Watts.

All these technological enhancements challenge the established order of brute power and diesel traction. With the UK government goal of eliminating diesel-only traction from Britain’s railways by 2040, there is a growing incentive to develop and deploy electric and hybrid traction. However, with green ambition out pacing electrification, and around half the UK network still without wires, the case for locomotives like the class 93 grows ever more compelling.

Author: Simon Walton

Simon Walton is RailFreight's UK correspondent.

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