20-02-2009, 08:34 PM
(20-02-2009, 02:46 PM)Jerry1237 Wrote: Okay, so effectively it is a legacy figure. That makes sense.
Some work I am involved in is trying to derive brake rates for freight trains of variable lengths/loads. The figures required, although they are the second line of defense, still are required to be realistic enough to stop a train, with a "normal" level of adhesion, in a predicatable yet operable distance. Hence, my hidden question was where is the engineering justification behind the value or was it a reasonable assumption from days of old. The answer appears to be the latter.
When we have some typical freight figures, I'll publish them here. They will make interesting reading based on theory backed up with practical testing.
Also interesting from an "effectiveness of TPWS" perspective. NR's policy is to asssume a 12%g emergency brake rate after intervention with only a very nominal brake build-up delay. Problem is we know that many passenger trains don't really make it and freight certainly don't. We do "look at" optimising the actual placements for 9%g but not proving more equipment than we'd need for 12%g but deploying it to more closely reflect reality. One of the issue is that the "Emergency" brake rate of certain trains (notably the red tiliting ones...) is significantly LESS than their usual braking rate where "clever" braking such as regenerative is employed rather than relying on friction brakes.
We also assess the risk based upon 4.5%g and determine whether the fitment previously determined is ALARP. However it is actually the brake build up more than the actual final brake rate that is most important from intervention at TSS (though not from OSS). TPWS was of course a "quick and easy cost-effective" solution to give GOOD protection in the MAJORITY of circumstances; it has proved itself great for that but could never be fully effective train protection.
Hence I'd be interested in seeing them but you might wish to ensure official clearance to publish first as they could just open up a whole new "can of worms" between TOCs/ FOCs and NR; at present the lid is on with only a few tails hanging out.........
PJW