(27-08-2010, 06:01 AM)jenni.joseph9 Wrote: Hi,
I tried to solve the question and got stuck up while calculating the strike in points. I am here with attachng the solved part for reference.
Can anyone please help to solve
You started OK for calculating crossing speeds and it was sensible to recognise that,whereas drivers might be able to see 1000m, to have adequate visibility to be able to determine in all weather conditions and lighting conditions that the crossing is not obstructed that some lesser distance should be set (better to explain WHY, not just quote a standard).
However you haven't considered both freight and passenger as requested and 9%g is a braking rate that even many passsenger trains wouldn't achieve particularly if you factor in driver reaction time and build up time; definitely no freight would achieve. Howver perhaps the freight may have a lower permissible spedd than the passenger on the line as a whole anyway.
You then say that the "crossing time" is the distance divided by time, but make it clear what you are calculating. This is the time that it would take for the train, when passing at constant speed (the maximum authorised crossing speed) the place at which they would otherwise have needed to be making a full brake application to be able to stop, to reach the crossing. I am not sure exactly why you have calculated this at this time as it wasn't specifically requested. However you are right that it is
part of the answer needed later since we will need to know how long it would be before a train reaches the crossing after the first warning of approach starts the sequence.
You are correct that you need to position AWB to give drivers adequate braking distance prior to the SSRB to respect the crossing speed from that position; again you need to consider both passenger and freight.
The thing that seems to be eluding you is the fact that you need to ensure that by the time the driver is observing the DCI and checking the crossing is clear of obstruction then the road user must have previously been warned by the level crossing light, audible and barrier sequence.
You should therefore calculate the time prior to the train reaching the SSRB needed to achieve this state of affairs and then add it to the time calculated earlier to ensure that a compliant warning time is given to the road user. From this you then work out the relevant distance that the train would be from the crossing when it should initiate the sequence and thus determine the strike-in point.
Remember when calculating this that:
a) the train will potentially be braking between the AWB and the SSRB
b) it is a double track level crossing and hence there is a Minimum Road Open Time requirement
Factor these into your calculations as well.
Beware of choosing a question just because it looks like Newtons Laws of Motion and expecting that school mathematics will see you through. Yes you do need to be able to do the calculations, but actually there will always be a need to demonstrate signalling knowledge and understanding in order to determine what to calculate. However for exam preparation it is good to attempt such questions as it is by doing so that you find out where you get stuck and can get some help. Things often seem easy until you actually try to do them, so well done for giving it a go.
