Railway operation and control
POINTS AND CROSSINGS
- Points, crossings, turnouts and cross-overs are arrangements by which different routes either parallel or diverging are connected
- Allows the trains to move from one track to another
- also used in marshalling and shunting work in station yards.
- Turnout is a simplest combination of points and crossings which enables one track to takeoff to another track
NECESSITY OF POINTS AND CROSSINGS
- Points and crossings provide flexibility of movement by connecting one line to another
- Also helps in imposing restrictions over turn outs
Heel divergence or heel clearance
- Distance between the running faces of the stock rail and gauge face of the tongue rail when measured at the heel of the switch
- Kept equal to flange way clearance plus tolerance for the wear plus the width of head of rail
- B.G Heel clearance = 13.7 cm to 13.3 cm
- M.G Heel clearance = 12.1 cm to 11.7 cm
- N.G Heel clearance = 9.8 cm
Flange way clearance
- Distance between adjacent faces of the stock rail and the tongue rail
Flange way depth
- Vertical distance between the top surface of the running rail (or stock rail) to the top surface of heel block used between the stock rail and the tongue rail
- Gradual widening of the flange way which is formed by bending or splaying the end of a check rail or wing rail away from the gauge line.
- This is provided to guide the path so that the flange wheels enter and leave the track smoothly.
- Device which provides two flange ways through which the wheels may move, when two rails intersect each other at an angle
- Flange wheels of the train jump over the gap from throat to the nose of crossing.
Requirements of good crossing
- The assembly of crossing has to be rigid
- In order to prevent the wear, nose should be made of special steel alloy
- Crossing body should be rigid and long
- Nose of the crossing should have some thickness, varying from 6mm to 8mm components of a turnout
Types of crossings
1.Acute angle crossing or V crossing
2.Obtuse angle crossing or Diamond crossing
- Angle of switch divergence
- Angle between running faces of the stock rail and tongue rail
- Switch angle= heel divergence/Length of tongue rail
Throw of switch
- Distance through which the toe of the tongue rail moves sideways (with heel of tongue rail as the centre of rotation) to provide a path for the desired direction over the turn out
- 9.5 cm for B.G.
- 8.9 cm for M.G and N.G.
- Point of intersection of the running face of the splice rail and the point rail is called the nose of the crossing
- The actual intersection of both the rails is called the T.N.C
- The nose of crossing provided in the rail in practice is called A.N.C
Distance between ANC and TNC
n = N x t
where n is the distance between ANC and TNC
N is the crossing number
t is the thickness of the nose of crossing
Crossing Number (N)
- Crossings are designated in terms of the distance required in spreading the point and splice rails by 305 mm.
- Spread is measured between the gauge faces of the rails and the distance is measured from theoretical nose of crossing.
- Angle formed between the gauge faces of the crossing
Methods to determine N
1.Right angle or Cole’s method
- Used by Indian Railways
- Main line and branch line form a right angled triangle with angle of crossing as α
- tan α = 1/N
i.e. N = cot α
2. Isosceles triangle method
- Main line and branch line is expressed in form of an isosceles triangle with angle of crossing as α
- sin (α/2) = 1/2N
i.e. 2N = cosec (α/2)
3. Centre line method
- The horizontal distance is N
- tan (α/2) = 1/2N
i.e. 2N = cot (α/2)
Switch lead (SL)
- It is the distance from the springing of the crossing curve to the heel of the switch, the distance measured along the straight
Different type of leads
- Switch lead
- Curve lead
Curve lead (CL)
- It is the distance from the springing of the crossing curve to the theoretical nose of the crossing, the distance being measured along the straight
Lead (L) = curve lead (CL)- switch lead (SL)
- It is the distance from the heel of the switch to the theoretical nose of the crossing, distance measured along the straight.