What the 50 kg/m Rail Section Is
The 50 kg/m rail, normally written P50, is a standard hot-rolled railway section covered by GB/T 2585-2007. Its theoretical mass of 51.514 kg per metre places it between the 43 kg/m and 60 kg/m profiles. It is the section most often chosen for industrial sidings, yard and depot track, metro depot lines and secondary main lines where traffic density and axle load do not justify a 60 kg/m rail. Because the profile is fixed by the standard, rails from conforming mills interchange at joints, and base plates, clips, fishplates and bolt hole spacings are all dimensioned around the same geometry. Rail steel grades used for this section, such as U71Mn and U75V, are designated in TB/T 2344-2012.
Profile Dimensions
The dimensions below are the ones used when checking a delivery against the purchase order and when designing base plates and joint bars.
| Parameter | Value |
|---|---|
| Rail height | 152 mm |
| Head width | 70 mm |
| Base width | 132 mm |
| Web thickness | 15.5 mm |
| Bolt hole diameter | 31 mm |
| Rail end to centre of first hole | 66 mm |
| Centre distance, hole 1 to hole 2 | 150 mm |
| Centre distance, hole 2 to hole 3 | 140 mm |
| Theoretical mass | 51.514 kg/m |
| Standard stock lengths | 12.5 m and 25 m |
Section Properties and What They Mean in Service
Centre of gravity above the rail base is approximately 67.1 mm, a value used in load distribution calculations for the rail section and for the pads and base plates beneath it. The second moment of area is about 2830 cm⁴ about the horizontal axis and about 450 cm⁴ about the vertical axis. The horizontal figure governs bending under vertical wheel load, so it drives deflection between sleepers and the resulting rail bending stress. The much smaller vertical figure governs resistance to sideways bending and to rail roll, which is why a 50 kg/m rail depends on the fastening system for lateral restraint on curves and on track with high lateral forces.
Head and Base Geometry
The running surface of the head is rolled with an inner radius of roughly 28-32 mm and an outer radius of roughly 68-72 mm. This two-radius crown spreads wheel contact stress and limits the growth of wear and rolling contact fatigue. Base edges carry a chamfer of about 1-2 mm, which avoids stress concentration during handling and stacking and allows the base to engage base plate shoulders without binding.
Supply Tolerances and Straightness
Typical supply tolerances for a P50 rail are plus or minus 0.5 mm on rail height, plus or minus 0.5 mm on head and base width, and plus or minus 0.3 mm on web thickness. Straightness is normally specified so that deviation does not exceed 0.5 mm in any metre and 4 mm over the full length. Tolerances and measuring methods vary between mills, so the acceptance level and the inspection method should be stated on the purchase order rather than assumed.
Shortened Rails for Curve Laying
Production lengths are 12.5 m and 25 m. For curve laying, shortened rails are supplied as 12.46 m, 12.42 m and 12.38 m cut from 12.5 m stock, and as 24.96 m, 24.92 m and 24.84 m cut from 25 m stock. Ordering the correct set of shortened lengths reduces the number of closed joints on a curve, limits field cutting and keeps the laid-up string within the planned joint positions.
Frequently Asked Questions
Q: What is the theoretical mass of a GB 50 kg/m rail?
A: 51.514 kg/m as defined by the standard profile, which gives approximately 644 kg for a 12.5 m rail and 1288 kg for a 25 m rail before end trimming.
Q: Which standard defines the 50 kg/m rail profile?
A: GB/T 2585-2007 covers hot-rolled railway rails including the 50 kg/m section. Rail steel grade designations such as U71Mn and U75V are given in TB/T 2344-2012.
Q: What bolt hole diameter and spacing does the P50 rail use?
A: The bolt hole diameter is 31 mm, the first hole is 66 mm from the rail end, hole centres are 150 mm apart between holes 1 and 2, and 140 mm between holes 2 and 3.
Q: Can a 50 kg/m rail be used with 60 kg/m rail fastenings?
A: No. Base plates, clip seats and fishplates are dimensioned for a specific profile. A 152 mm high, 132 mm wide base cannot sit correctly in a seat designed for a 60 kg/m rail, so the fastening set must match the rail section.
Q: How is straightness of the rail measured on arrival?
A: Check the gap under a 1 m straightedge at several points along the rail and confirm the maximum deviation in any metre and the maximum deviation over the full length against the levels agreed on the order.
Q: Why are shortened rails supplied for curves?
A: Shortening compensates for the difference in arc length between the inner and outer rail of a curve and for laying-up tolerances, which keeps joints in the intended positions and avoids on-site cutting of a standard length.