Exercise 1A, Appendix W of the Study Pack

[reuben]

I have included my calcs for question 1 below I have a query is the maximum permitted spacing for 3 aspect SBD + 33%?

Convert Line Speed into ms-1: 80 x (1609/3600) = 35.75ms-1
say 36ms-1

Find Braking Distance using the following formula BD = u2/2b, where b is the deceleration rate (usually given in the notes, assume 0.5ms-2 if not stated), and u is the train speed.

BD= (36)2 / (2 x 0.72) = 1296 / 1.44
BD = 900m

Maximum permitted spacing is SBD + 33%

SBD = 1258m + 33% = 1673 m

Sighting distance is 12 sec at headway speed = 12 x 36 = 432 m

Overlap = 180 m

Hd3 = (2B + S + O + L)

Hd3 = ( (2 x 1673) + 432 + 180 + 100 )
Hd3 = 4058 m

Ht3 = Hd3/V
Ht3 = 4058/36= 113secs round up to 115secs

Answer:
Headway time for 3-aspects at maximum permitted spacing = 115 seconds


Also is it best to submit my answer to the appendix as a job lot or can I submit them in this thread as I o them one at a time?

From my perspective I still have design for the commissioning starting on 24/12/10 to sign off as CRE and there are a million and one things to do to try to stop the whole job going very pearshaped; therefore I shall not have time to look at until 2011 so I suggest that you do as a combined lot.

Yes 133% braking is a typical maximum spacing, although in some circumstances this can be exceeeded it is a suitable value to utilise in this context.

Merlin

Good calcs.

Not sure where you got the figure of 1258m from? - you just calculated braking distance on the previous line as 900m. It appears that you've added ??% and 33% to the minimum safe braking distance.

The current railway group standard GK/RT0034 states the (typical) figure of 33% allowed overbraking. However, the figure used in the past was 50%, and there is a "temporary non compliance" issued against the standard, allowing the old 50% figure to remain. In general, using the 50% figure gives you more flexibility in positioning signals, so use this to your advantage in the exam!

Looking at your equations, you have used quantities "BD", SBD" and "B" at different times, not clear which is which.

regarding the equation for 3 aspect headway time, i always think it's worthwhile drawing a sketch (much quicker on paper than on a website!) showing the position of the 2 succesive trains, and all the dimensions. This justifies the equation, instead of just plucking it out of the air.

In this question, you can save a little time by directly adding the sighting allowance as a time: looking at the maths, you've multiplied 12 by V, then added this to a figure which you then divided by V. it would be quicker to initially ignore sighting in the distance equation, and simplly add 12s to your calculated answer.

reuben