How does vacuum braking switch back to air brake? What is the difference between the "passenger" and "truck" modes?
Its regulating the brake timings. Up to 60mph use goods, above the passenger brake mode. In air braked condition, isolate the exhauster since you dont need the vacuum pump. Also be sure to empty the chambers by venting the relay valve. In vacum braked condition turn on the exhauster and alternate compressor changeover. (The vavuum is air controlled so you still need a compressor) The traction cutout is something you can ignore if you are new to railroad stuff, rather learn more about the brake systems.
One option is just to leave the brakes in vacuum mode. In this mode, the 37 will still control air brakes as normal, as that is the primary braking system, and vacuum is secondary. After switching from vacuum to air brakes, the vacuum brakes will still be "charged" and may be impossible to release conventionally. You must therefore discharge them by using the white chamber-release valve, which on the 37 is on the secondman's side of the cab, under the handbrake wheel. Leave this valve open until the vacuum in both the pipe and chamber sides drops to zero; you should notice a gauge on the driver's side with two needles for this purpose. The difference between "passenger" and "goods" brake modes, both air and vacuum, is in how quickly the locomotive brakes respond to changes in the brake pipe pressure. Passenger trains are expected to require quick responses, and to have sophisticated couplings which minimise slack between carriages. Freight trains are expected to have simpler couplings and much more total coupling slack (at least in long, heavy trains), so the brakes may be set to "goods timings" so that the brake force is introduced and removed more gradually. Most freight wagons have a timing valve on them somewhere, but in TSW they will tend to default to "goods" mode. The locomotive has this timing valve because it may be used for both types of train.
Note that one doesn't have to use goods timing with a freight train: if it's relatively light and short it's unnecessary, and provides sluggish response (both application and release) compared to passenger timing. For example, there's no need to use goods timing with an RHTT.
The brake mode you use is determined by the wagons you're coupled to, simple as that. It needs to either be all vacuum braked wagons or all air braked wagons. Once you know which one it is, set the mode and off you go. Vacuum brake works by using an "ejector" fan to create a vacuum in the brake pipe, once 21inhg of vacuum is reached, the brakes are released. To apply brakes, the vacuum is reduced (by letting air back in the pipe) and this causes the brakes to come on. If there's ever a snap and the brake pipe splits, that instantly destroys the vacuum and the brakes come on hard. Air brake works the other way around, a compressor pumps the brake pipe up to e.g. 5 bar of pressure. At 5 bar, the brakes are released. To apply brakes, the brake pipe is allowed to lose some air pressure (through the miracle of "a small hole"), as it drops, so the brakes will be applied. If the trains breaks, and splits the brake pipe, the air pressure is instantly vented to the atmosphere and this results in the brakes applying. Both are fail safe - but creating vacuum is inherently harder than creating pressure, you'll find vacuum brakes are less responsive in terms of how fast they react, or to get the vac pipe back to the 21 inches of mercury to get the brakes fully released, compared to air brakes which can work faster and support more wagons / longer trains. The difference between Freight and Passenger is down to the rate at which brakes are applied. As the brakes apply and release, there is an inherent difference in pressure between the front and the back. In an air braked system, releasing brakes means pumping up the pipe - so the front of the train is always going to have a higher pressure than the back until it all equalises. Releasing brakes means dropping the pressure, and trhe same applies (in so far as lower pressure in front...) since the "little hole" is at the front of the train. So what happens is that you will command an application of the brakes, and without any regulation, you could find (depending on various factors like the number of wagons and types, length of brake pipe etc) that the loco is at the desired brake pipe pressure rapidly - so its brakes are applied as you want them - but none of the other brakes on the train are yet, gradually as the brake pipe pressure drops, more and more of the train going backwards (imagine a "wave" going down the length of the train applying brakes). Doesn't matter though - because you're probably sliding the front of the train now - it was like you had applied only the loco's brakes, or maybe the loco and the first wagon and the rest are pushing and yelling excitedly "ON! ON! ON!". Throw in "freight" timing, and what happens now is that the rate at which the brake pipe is allowed to change pressure is reduced. So now, you're going down the hill, you apply your brakes, and you'll see the brake pipe drop much more slowly to meet the desired pressure - what this is doing is keeping the brake pipe pressure much more evenly balanced down the length of the train and making it so that you don't get the above situation, all wheels braking much more in parallel. For me then, it's heavy trains, long trains and "any time you are in any doubt at all" that you put it in Freight mode. Freight mode means slower rates of braking, it means you need to plan ahead more and give yourself more time to stop, but you're ultimately in more control of that train. Passenger mode if you're confident - usually with modern longer length wagons for example, or with the lighter loads, you can get away with much more rapid braking afforded in passenger mode. The fundamental mistake people often make, because it's really not obvious in the slightest, is that the brake pipe pressure is not a linear value where the front and back are always identical. In some US trains that are 1 or 2 miles long, you get much more extreme versions of this than on UK trains, but the principle is the same. Also remember, to further confuse this, is that the "brake pipe" reading you get in the cab is ONLY your locomotive. If you have a train with end of train device fitted (mostly in the US) then you can see the rear brake pipe pressure from that usually and then this becomes more clear - but on any UK train you're stuck with just the front brake pipe pressure, and an educated guess as to the rear based on what you've done, what you're seeing in the gauges you CAN see, and what you're feeling from the train. If you're driving a freight train and you don't have the option to select brake timings but you think you're running into this problem where the front ends up braking more than the rear by too much - then you can just manually achieve the same result but applying your brakes gradually. Small application, wait, small application, wait etc, that keeps the pressure differences much narrower and minimises issues greatly. Hope this helps. Matt.,
