Rail expansion

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Thermal expansion can cause track misalignment, typically referred to as "sun kink". Track must be laid with gaps between adjoining rails to reduce the likelihood of sun kinks.

The table below provides recommended gaps for the ambient temperature when the track is laid. Gaps are indicated in fractions of an inch and temperature is in Fahrenheit. Note that the scale of the rail has no bearing on the size of the gap required.

IBLS Rail Gap Standard
Rail Type Rail Length (inches) 0°F 20°F 40°F 60°F 80°F 100°F 120°F 140°F
Aluminum (6063-T5) 120 1/4 3/16 5/32 1/8 3/32 1/16 1/32 no gap
Aluminum (6061-T5) 120 1/4 1/5 1/6 1/8 1/10 1/16 1/32 no gap
Steel 120 1/8 1/10 1/13 1/16 1/20 1/32 1/64 no gap
Aluminum (6063-T5) 240 2/5 3/8 5/16 1/4 3/16 1/8 1/16 no gap
Aluminum (6061-T5) 240 1/2 5/13 1/3 1/4 1/5 1/8 1/16 no gap
Steel 240 1/4 1/5 1/6 1/8 1/10 1/16 1/32 no gap

Rail creep

From Chaski.org:

Many don't realize there's "rail creep" in our hobby rails as well. Seen all too often where rail gaps near the bottom of a grade close up over time. Even had one guy complain there were no rail gaps. Well, the rail gaps were there when the track was installed...but closed up over time.
Trains traveling downgrade are braking...and pushing the rail in front of the train. Trains traveling upgrade...are trying to pull the rail under the train. There's an old joke about the track gang foreman removing a section of rail at the bottom of the hill...and moving that back to the top of the hill!
Would add....rail creep and rail expansion/contraction due to temperature variations are two different things. Carl B.