Building the Mallet 'River Brue'

see also 'River Brue' overhaul

'River Brue' hauls an eight coach holiday special from Barnstaple - click on image to enlarge

the 'Mad Mallet  'River Avon'
The 'Mad Mallet' began as a joke on the Lynton and Barnstaple forum about articulated locomotives that could been built. I was stuck with my work and had a spare hour to get bored and watch telly, so I drew an L&B Mallet on my laptop with the company of Judge Judy. Rather embarrassingly, I fell in love with the design. Of course, the original design had to be changed, as the rear bogie made it far too long.  Probably the design would have been better served if pannier tanks had been used which would have given access to the steam pipe.

line drawing of 'River Brue' - click on image to enlarge
I have always rather liked Mallets. As a kid I had the privilege of seeing Challengers blast through the Rockies. You never forget a thing like that! Garratts might be a better design but I have never warmed to seeing locomotives with two backsides.
The Mallet has its origins in France. The design minimised the number of flexible high pressure steam pipes and limited their movement by having the rear power bogie fixed to the boiler so only a small flexibility was needed. The design really favoured the use of compounding. The system reused the low pressure steam to operate a secondary drive using much larger cylinders. By the time this engine was built (1920s) greater efficiency would have been extracted by the use of super heating.

Mallet locomotives were used with success on a number of European lines, mostly narrow gauge, but really came into their own in the USA. Here they were a huge success as they permitted the Americans to have the biggest in the world without needing any imagination of their own!
The heavily graded line of the L&B required double heading if any sizable trains were mustered: an expensive business. Had traffic been greater, it would have been quite possible that Eastleigh would have contemplated building more powerful traction units that still had to operate within the loading gauge and go round the numerous tight curves.
The L&B benefited from the development experience of Welsh narrow gauge railways. The one mistake made was the relative lightness of the track. The only way to get more power would have been to have more wheels. The Mallet could easily have been considered.

the first Mallet, 'River Avon'
The first Mallet we built was based on the terrible Roco outside frame chassis.  Performance continued to deteriorate and the decision was taken to re-chassis this one too.
The second Mallet, 'River Brue' represents the second Eastleigh built unit and incorporates several improvements after the experience gained with 'River  Avon' including larger cylinders and externally mounted low pressure steam pipes to the low pressure cylinders. A larger boiler is incorporated and enlarged water tanks. The design represents the maximum size that can operate within the L&B loading gauge.
It was originally intended to base it on the new N Drive Productions outside frame six coupled chassis, two of which are required. Sadly, production delays have prompted us to purchase two Bachman/Grafar 08 class diesel no 371-015 which fortuitously became available and were delivered on Christmas Eve.

the plan is to use two of these units.
The chassis has a wheelbase of 24mm, a footplate length of 108mm and a motor width of 8mm.

cylinders and valve gear

So the first job, while awaiting the delivery of the chassis has been to build the low pressure cylinders and attach the valve gear.

29 Nov. - low pressure cylinders and gear - click on image to enlarge

Operating experience has shown that the etched valve gear is perhaps a little too heavy for the Grafar chassis. The operation is less smooth than when using Roco valve gear...(obtained from Howes at about 13).


If one has built Backwoods Manning Wardles in SR condition, one lands up with quite a lot of spare etches, including the original cab sides. These are used in this model soldered to nickel silver sheet cut to size.

30 Nov. - the completed cab sides - click on image to enlarge and for dimensions

30 Nov. - start of body construction

30 Nov. - start of body construction

30 Nov. - start of detailing. The loco will be fitted with working headlights so a steam generator has been added
 - click on image to enlarge

1 Dec. - start of detailing. all the body detail complete, (handrails, dome, sandpots and chimney to be fitted later). Pilot now complete - click on image to enlarge

more or less scale 'hinge' for front bogie

the 'box of bits' - click on image to enlarge

wiring diagram
Above is the wiring diagram for the loco. The larger DZ143 chip will be mounted in the cab roof and will operate both bogies.

'representational' firebox and backhead but more than adequate when hidden in the small cab
click on image to enlarge


The Grafar chassis is easy to dismantle and the first was cut back and the high pressure cylinders fitted with epoxy. Great care is needed to position the cylinders to ensure that the correct movement of the valve gear is possible. They have to be as close to the front crank as possible. The rear coupling rod pin was removed and the hole opened out slightly on the crank to accommodate a brass wire pin.

The chassis is well made and the drive is on the centre wheel. This does mean that additional strain is taken on the rear axle with the heavy valve gear. I suspect that eventually, excessive wear will take place.  For this reason, once I am happy with the setup, this axle will be cleaned and treated with silicone releasing agent and reset into the slot which will be filled with plastic metal which will hold the axle in place far better.
The chassis runs very smoothly but is far wider to the cranks than the N drive chassis, which forces the valve gear to be further out than I would like.
The return crank and crank pin were soldered together in a jig and then set into the crank with epoxy. They were left to harden for 24 hours. The return cranks need to have 2 mm centres. The return rod was then fitted and retained with a small washer and a touch of solder. The high pressure cylinders were recovered from one of the scrap Roco chassis.
The front bogie needed more cutting back and attachments for the footplate had to be reconstructed. Take care not to overheat the chassis if you cut back with a Dremel tool.
If the loco is to be fitted with DCC, the connections from the pickups have to be separated from the motor at this time.
The electric motor can be pressed down to release the worm gear from the drive. Free rotation can be checked in this condition.

The rear chassis, cut down and with cylinders and valve gear fitted. The return rod had not been retained at this time. - click on image to enlarge

The two chassis are united for the first time. - click on image to enlarge
The flexible front bogie is hinged at the rear (some play is needed). At first, I employed a rubbing plate at the front but soon found that it was better to weight the chassis to the maximum and not support the front of the body. Some weighting is required at the rear of the body to obtain balance.

Great care is needed to ensure that all feet are firmly on the floor. Set up the front bogie height on a sheet of glass. The weight of the bogie at the front, if the loco is removed from the rails, is taken by the flexible blast pipe which is retained under the pilot with a soldered washer. This engages with a slotted box which is soldered to the smoke box.



The flexible blast pipe and the rubbing strip on the front bogie
The angled slotted box which engages the blast pipe. There is sufficient clearance to allow for movement. The bogie swing is set up for a maximum of 11 1/2 " radius, (shown here) - click on image to enlarge
I have discovered that one wheelset is out of back to back and have ordered a replacement.

The loco after successful testing still without pony trucks. The prominent  lagged low pressure pipes to the front cylinders are clearly visible. - click on image to enlarge
After a little fettling, the loco runs very smoothly at low speed and is of course very powerful indeed. The two chassis have been weighted with lead and some time was taken balancing weight in the body. After another very long day, the pony trucks were fitted and chassis detailing which include brake rodding, injectors etc.

nearing completion - click on image to enlarge

nearing completion - click on image to enlarge

the two power bogies - click on image to enlarge

A comparison with River Avon. Brue has a slightly bigger boiler and enlarged water tanks. Bit of a beast really! - click on image to enlarge
So the model (minus the motors) is popped into the washing machine and then etched and sprayed in black. Shortly it will be off to Tony Freestone for panelling and lining. Peter Blackham has produced the necessary transfers for me. This model will be turned out as 'fresh from the paintshop'.

click on image to enlarge

'River Brue' ready for service but at this time still awaiting name and works plates - click on image to enlarge

5 mile overhaul

River Brue ran in exhibitions for just over 5 miles before beginning to show signs of unreliability. It had become increasingly rough in operation and was starting a distinct 'waddle'. There has been concern that the Bachmann 08 chassis might develop similar faults to the Roco chassis. Upon dismantling, acceptable wear was found in the main axle boxes but it is clear that the heavy nickel silver valve gear puts too much strain on the chassis.

A decision was made to replace the chassis with two new Bachmann units fitted with ROCO valve gear. These are quite reliable and replacement is easy. I have to say that these days I remove the brake detail as they get in the way of easy removal of the keeper plate. The detail is effectively invisible anyway.

The new rear motor unit