OVERLAPS versus CATCHPOINTS for Simultaneous Arrivals.
To allow simultaneous arrivals at Crossing Loops it is necessary to have either
* an overlap of perhaps 300m past the starting/departure signal;
* a catchpoint or run-off lead past the starting/departure signal;
* an intermediate home signal protecting an overlap;
* some kind of ATP or ERTMS that regulates the speed of trains entering the crossing station on main or loop. No examples so far.
If Up and Down working is applicable you need two catchpoints past the respective Starting Signal
If Main and Loops working is applicable you may need four catchpoints.
Sometimes the catchpoint is implemented with a full lead, such as
* Thornleigh ETTT.
* Shellharbour Junction Up Loop
Crossing loops with U&D catchpoints include
* Albion Park.
Crossing loops with catchpoints in down direction and intermediate home signal and overlap in up direction
Crossing loops with catchpoints in both directions on Loop Line where there is no platform.
Crossing loops with overlaps in both directions allowing simultaneous arrivals include:
* Bells Gate
* Bylong ? - double length loop with intermediate signals and level xing in middle.
Other states are not so restrictive.
The Home/Arrival signal and preceding distant signal indications have a bearing on whether Sumultaneous Arrivals are possible. If they show Low Speed then No. If they show "medium" then maybe. In NSW a Low Speed guarantees that the Loop is unoccupied. In VIC the low speed does NOT guarantee that the Loop is unoccupied. Etc., etc.
In QLD, a home signal entering a main or loop with no overlap shows a Flashing Yellow which means no overlap.
In NSW, a flashing yellow has a contradictory meaning, namely next signal clear for straight or turnout.
DISADVANTAGES OF NON-SIMULTANEOUS ARRIVALS.
For the sake of the argument it will be assumed that the Up train arrives first and takes the loop, while the Down train arrives second and takes the main. Also NSW nomenclature is used.
The Down train will not lose any time if the Up is locked away and stationary before the Down train sights the Down Distant signal showing Green from its sighting distance. In this case the Down Trains passes through the Main and loses no time because of the Cross. The Up train will then be able to resume its journey in the shortest possible time.
If the Up train doesn't enter the loop in time, and noting any low speed loop points, then the Down train will receive a Yellow at the Down distance, and will have to reduce speed expecting to have to stop at the Down Home signal. Once the Up train is fully in the Loop, then the Down Train will find the Down Home and Down Starting signal both at Clear, and it can restart from a stationary position. Once the Down train has passed the Loop points at the Up end, the Up train can resume its journey, losing what ever time is lost because of low speed turnouts.
The higher the speed of the loop points the better.
Time is also lost recharging the air brakes on conventional trains. Electronically controlled pneumatic brakes (ECPB) on some trains improve braking performance. Acceleration and braking rates need to be adjusted for gradients and the empty or full state of the load. Speeds need to be adjusted for speed limits on plain lines and through points including trains lengths. Separate calculations need to be done for short passenger trains and long freight trains.
The equations of motion need to be applied to give numerical values to the movements. These include:
V^2 = U^2 +2.a.s .
s = (V^2 - U^2)/2.a
V = U + a.t
a = (V - U) / t
t = (V - U) / a
It goes without saying that calculating the passage time through crossing loops is complicated and needs to be done with a spreadsheet or a specialist program like MTRAIN.
ADVANTAGES OF SIMULTANEOUS ARRIVALS.
In this case, the second train to arrive gets a medium aspect (Flashing Yellow) at the Down Distant, and a Caution aspect (Yellow) at the Down Home and is prepared to stop at the Down Starting Signal until the Up train is fully in the Loop, in which case the Starting Signal will clear. Once the rear of the Down train clears the clearance post near the points at the Up end, the Up train can proceed on its way though the low speed points.
The higher the speed of the loop points the better, especially for departing trains.
For equations of motion see https://en.wikipedia.org/wiki/Equations_of_motion
After 10 years of development ARTC and Lockheed Martin are about to commission their Advanced Train Management System (ATMS) in a big way. A trial section is between Port Augusta and Whyalla. ATMS replaces lineside signals with radio and satellite "virtual signals", with improved safety, speeds, and headways. Ability to regulate the speed of trains continuously at crossing loops, would probably make "simultaneous arrivals" available everywhere. Signals are replaced by "Main Line Indicators" (MLI). Level Crossings are also controlled and supervised. See https://www.artc.com.au/projects/atms/
ATMS (Advanced Train Management System)See 2012 [size=2]https://www.railpage.com.au:80/f-t11327619-0-asc-s75.htm[/size]
ATMSSee 2014 https://www.railpage.com.au/f-p1901093.htm ATMS
See 2015 https://www.railpage.com.au/f-p1932580.htm TCMS
See 2015 [size=2]https://www.railpage.com.au/f-t11382084.htm[/size]
See 2020 [size=2]https://www.railpage.com.au/news/topic-40/[/size]
See 2020 [color=#0e7744]http://www.railpage.com.au/news/s/rapid-adoption-of-atms-key-to-freight-rail-competitiveness[/color] ATMS