Clan Line is not air-braked. The loco is still steam-braked, and the tender is still vacuum-braked. We also have a mechanical hand brake which just operates the tender brakes. However, we have modified our engine to allow it to operate air-braked trains. This was done to allow us to haul the British Pullmans, and other charter rolling stock. All three braking systems are linked together, so, under normal situations, the driver just has to operate one brake control.
The steam brake operates on the driving wheels. Operating the steam brake valve in the cab allows steam to enter the steam brake cylinders which are situated between the loco frames. This causes pistons in the brake cylinders to move, which, in turn, move the brake rigging. This forces the brake shoes to be applied to the outside of the tyres of the driving wheels, thus slowing the engine down. When the steam brake is released, springs within the brake cylinders cause the pistons to return to their usual position, and the weight of the brake rigging allows the brakes to come off.
The vacuum brake operates on the tender wheels. It used to also operate the brakes on the train, and could still do so, if necessary. A vacuum (or, more accurately, reduced air pressure) acts on cylinders on each vacuum-braked vehicle. This vacuum holds the brakes off. To apply the brakes, air is admitted to part of the system, which causes pistons in the vacuum cylinders to move, and apply the brakes. To release the brakes, the vacuum is recreated, and the pistons return to their original position. A vacuum is measured in “inches of mercury (Hg)”, in the same way as barometric pressure. The normal running position, with the brakes off, should be 21”Hg (except for Great Western engines).
The vacuum is created by the ejector. This uses steam, passing through a converging cone, to create a drop in pressure. As the steam passes through the cone, it speeds up. This means that its kinetic energy increases. Because of the conservation of energy, this energy has to come from somewhere, and it comes from the potential pressure energy, so the pressure drops. The same principle is used in the injectors and the blast pipe. It is also what makes an aeroplane fly.
We actually have two ejectors, which are known as the small ejector, and the large ejector. They are operated by the driver, who usually has the small ejector turned on all the time. When hauling a vacuum-braked train, the driver could also use the large ejector if he wanted to get the brakes off particularly quickly. The same control lever is used to put the brake on. Putting it all the way up brings on the large ejector, and all the way down puts the vacuum brake on. The normal running position is in between the two.
There is a connector, usually known as a vacuum bag, at the ends of each vacuum-braked vehicle. At each end of the train, these are placed on dummies to seal the vacuum. Coupling up includes joining these connectors. Before moving off, a brake-continuity test is done. This involves releasing the vacuum at the end of the train, and ensuring that the drop in vacuum is seen in the cab. If it is not, then there is a blockage somewhere, and some of the brakes on the train would not work. If a coupling fails, and the train parts, the connectors would break, the vacuum would be lost, and the brakes on each half of the train would come on. Both parts of the train would come to a stop, which is a big advantage of the continuous brake.
The vacuum brake is connected to the steam brake. As the vacuum drops, the steam brake comes on proportionately. When this happens, the steam brake valve is moved by the vacuum. The driver doesn’t have to worry about the steam brake.
There is quite a skill in operating the vacuum brake. When applying the brake, the driver can’t just put the brake lever in one position and leave it – he has to keep adjusting it to keep the vacuum at the desired level. There is also quite a delay in any adjustment of the brake having an effect on the back of the train. Stopping a long train in the desired position takes some practice.
The air-braking system is a lot more complicated. The basic operation is quite straight forward, but a lot of the complication is involved in dealing with possible fault situations.
The principle is similar to that of the vacuum brake, except that we use air pressure instead of a vacuum. In normal running, the brake pipe is charged to 72½ psi (pounds per square inch), which is 5 bar (atmospheres). This means that, to exert the same force on the brakes, we can have much smaller brake cylinders throughout the train. It also means that the brakes react to the brake lever much more quickly.
There are two pipes running throughout the train – the (red) AABP (Automatic Air Brake Pipe), usually known as the “brake pipe”, and the (yellow) “main res” (Main Reservoir). It is possible to operate without the main res pipe – this is known as “single piped” – and some freight trains still do, but it takes longer to release the brakes this way.
The main res pipe is charged to 100 psi, and this is used to replenish the reservoirs throughout the train. The brake pipe runs at 72½ psi when the brakes are off. The first stage of applying the brakes, known as “first application”, or “initial”, drops the air pressure to 65 psi. The next stage, known as “full service”, drops the pressure to 48 psi. The brake can be adjusted to anywhere in between these two settings, which will adjust the braking force accordingly. There is also a setting of “emergency”, which drops all the air from the brake pipe. This doesn’t bring the brakes on any harder than full service, it just brings them on quicker.
A break in the brake pipe causes the brakes on the whole train to come on, in the same way as with vacuum. A break in the main res pipe is sealed by a valve in each connector, so that we don’t drop all the air from all the reservoirs throughout the train.
The first thing that we had to do to add air braking to Clan Line was to add a steam-powered compressor. This is situated at the rear of the tender, behind a pair of doors. It takes up a little bit of water space, but gives us good access without affecting the appearance of the loco. It is the exhaust from this compressor that produces the steam that rises from the rear of the tender. There are three tanks on the back of the tender, where the vacuum tanks used to be (they are now between the tender frames). The compressor pumps these tanks up to 140psi before the governor cuts in. There is also a safety valve which vents at 150psi. A pipe takes the air from the tanks, via a flexible connector, to the loco.
The supply comes to a reducing valve, the FVF2, which drops the pressure to 100psi, and on to the driver’s brake valve, an M8A. This is the valve that supplies the air, at the required pressure, for the brake pipe. All quite simple, really.
However, there is a bit more to it than that. The first thing is AWS (Automatic Warning System)/TPWS (Train Protection & Warning System). This needs to be able to drop the brake when required. The valve that does this is the EP (electro-pneumatic) valve, also known as the Baldwin valve. If TPWS/AWS needs to apply the brake, it drops the electrical power to the EP valve which, via a timing reservoir, slowly drops the brake, bringing the train to a controlled stop.
If the compressor fails, we need to avoid exhausting all the air in the tanks, so we cut off the air supply to the main res. This is done by the brake feed cut off valve. Associated with this is the duplex check valve. If the main res drops below 80psi, either because of a break in the yellow pipe, or because the compressor has failed, it brings the brakes on, in a controlled manner.
There are also two emergency valves. If the worst happens, and the crew need to evacuate the footplate in a hurry without being able to activate the brakes, there is a valve on each side that the crew can access on their way out to dump the brake. Let’s hope that we never need those two valves.
We also have to cope with an AWS/TPWS failure, so there is the option to run with AWS/TPWS disabled. We also have to be able to get the brakes off without main res, (if we were assisted by a single-piped loco), so there is the AFT (assist failed train) cock which, effectively, bypasses the duplex check valve.
All the above valves are not really designed to operate in the environment that exists on the footplate of a steam locomotive, so the late John Adams designed and built a metal box, situated under the cab floor, which contains and protects most of these valves.
There is another valve, the air vacuum relay valve (known as the DV2) which connects the air brake to the vacuum brake. This is a proportional valve, and, as the brake pipe pressure drops and rises, it causes the vacuum to fall and rise appropriately. As this happens, the steam brake also acts accordingly. If necessary, this valve can be bypassed, so that we can operate purely on vacuum. However, as AWS/TPWS operates on the air braking system, we would lose this. When we went to the Mid Hants gala a few years ago, we still had to have the air pumped up, as they have AWS. The drivers then had the option to use the vacuum controls or the air controls. Some chose one, and some chose the other. When we went to the Bluebell recently, as they don’t have AWS, we isolated the DV2, and so didn’t need the compressor running.
When running on the main line, we can still haul vacuum-braked trains, as long as the compressor is turned on, and the air-brake valve is in the running position. However, our support coach is purely air-braked, so we would have to leave this behind.
All three of these braking systems operate at different rates, both on applying the brakes, and releasing them. Therefore, it is difficult to get them balanced. We have our system set up so that the vacuum brake, and, subsequently, the steam brake, are only applied when the air brake is applied beyond the initial position. As well as saving our brake blocks, it means that the train doesn’t bunch up as the brakes are applied, and doesn’t snatch when the brakes are released and power is applied. It’s not perfect, and it is possible, if the brakes are released too quickly and power applied, for the tender brakes to still be applied, thus causing wheel flats. We can also have problems when running just engine and coach, as the coach brake will be applied first, possibly causing these wheels to lock, and giving us wheel flats.