What the 50 kg/m Designation Means
The figure in the name of a rail is its nominal mass per metre, not its height or its width. The 50 kg/m rail is the mid-weight profile of the Chinese heavy rail family and is specified together with the 38 kg, 43 kg and 60 kg profiles in the national rail standard, GB/T 2585, which also fixes the section dimensions and the mass tolerance. It is a hot-rolled carbon-manganese rail supplied for mainline track at moderate speed and axle load, for station and siding track, and for industrial and crane installations where a heavier 60 kg/m rail is not justified.
Section Dimensions and Mass
| Size | Rail height (mm) | Bottom width (mm) | Head width (mm) | Web thickness (mm) | Mass (kg/m) |
|---|---|---|---|---|---|
| 38 kg | 134 | 114 | 68 | 13.0 | 38.730 |
| 43 kg | 140 | 114 | 70 | 14.5 | 44.653 |
| 50 kg | 152 | 132 | 70 | 15.5 | 51.514 |
The 50 kg/m section combines a 152 mm section height with a 132 mm foot width and a 15.5 mm web. The increased web thickness relative to the 43 kg profile is what gives the rail its higher bending stiffness and its tolerance of heavier axle loads, while the 70 mm head width keeps it compatible with standard fishplates and fastening hardware in the same family. Mass per metre matters directly in procurement, because rail is bought by weight but laid by length: a 12.5 m or 25 m standard length of 50 kg/m rail weighs about 644 kg or 1288 kg respectively, which drives crane and wagon selection on site.
Steel Grades and Delivery Condition
Rails in this family are produced from carbon-manganese rail steel, with U71Mn the conventional grade for general track and higher-hardness grades such as U75V specified where traffic is heavier or where curve wear is a concern. The rail is delivered in the hot-rolled or head-hardened condition depending on the grade and the contract, and the technical delivery conditions - chemical composition limits, tensile requirements, hardness, straightness and permissible surface defects - are set in the relevant rail standard and in the purchase specification. Buyers should state the grade, the delivery condition and the applicable standard on the order, because a rail that meets the dimensional table but not the grade requirement cannot be welded to the same procedure as the rest of a line.
Tolerances, Straightness and Camber
Dimensional tolerance is what separates a rail that can be laid on a running line from one that suits industrial track. The critical items are section height and web thickness tolerance, head symmetry, foot flatness, longitudinal straightness measured over a defined chord, and end squareness. Straightness is normally measured both in the vertical and in the horizontal plane, and it is also specified at the rail ends, because end deviation is amplified at a fishplated joint. Rails that fail the vertical straightness limit can be straightened within defined limits, but a rail with a twist or a bent end is generally cut back before it is used.
Applications and Selection
The 50 kg/m rail is the practical choice for mainline track where speeds and axle loads are moderate, for station, siding and depot track, and for industrial installations such as crane runways where the load is high but the running speed is low. It is pooled with the 43 kg and 38 kg profiles where a mixed system is already installed, which keeps fishplate, clip and sleeper hardware interchangeable. Where axle loads exceed the design range of the profile or where speeds are high, the heavier 60 kg/m rail with the corresponding fastening system is the correct selection, and mixing profiles within a single continuously welded string is not acceptable practice.
Ordering, Cutting and Drilling
Orders should define profile, grade, length, quantity, end finish and the standard, and should state whether the rail is required for welding into strings or for fishplated joints. Drilling for fishplates is carried out with the hole pattern and diameter specified for the joint bar being used, deburred on both faces, and positioned from the rail end within a defined tolerance, since hole position controls the joint gap. Cutting is done by saw or abrasive disc rather than by thermal cutting, because a flame-cut end leaves a hardened and cracked edge that cannot be welded without removing a substantial length. Rails that will be flash butt welded are normally ordered with a defined end condition and without pre-drilled holes at the weld ends.
Frequently Asked Questions
Q: What does 50 kg mean on a rail designation?
A: It is the nominal mass per metre. The 50 kg/m rail is a 152 mm high section with a 132 mm foot, 70 mm head and 15.5 mm web, and its nominal mass is 51.514 kg/m as listed in the section table of the rail standard.
Q: Which standard covers 50 kg/m rail in China?
A: GB/T 2585 covers the hot-rolled rails for railway use in this size family, including the 38 kg, 43 kg and 50 kg profiles and their section dimensions and mass. Technical delivery conditions such as grade, hardness and straightness are set out in the rail standard and in the purchase specification.
Q: How much does a length of 50 kg/m rail weigh?
A: At a nominal 51.514 kg/m, a 12.5 m rail weighs about 644 kg and a 25 m rail about 1288 kg. Handling equipment and wagon capacity are normally selected from those figures plus the tolerance allowance.
Q: Can 50 kg/m rail be welded into continuously welded track?
A: Yes, by flash butt welding in a plant or in the field, or by alumino-thermic welding for closures. The grade and the end condition have to be declared so that the welding procedure can be qualified for the actual steel.
Q: When should a heavier rail be used instead of 50 kg/m?
A: When axle loads, speed or curve severity exceed the design range of the profile, or when the line is part of a network that already runs 60 kg/m rail. Mixing profiles within one welded string is not acceptable because the running surface and the joint geometry do not match.
Q: How should rails be cut on site?
A: By saw or abrasive disc with the end square and deburred. Thermal cutting hardens and can crack the rail end, which then has to be cut back before welding or jointing.