The standard Plarail curve is a 45 degree, eight-to-a-circle curve with a radius of approximately 215mm to work well with the standard straight rail. At different times, a few different 90 degree curves have been sold, either as more stable curves usually for elevated bridges or in different radii, seemingly either to keep piece count down or to prevent the odd dimensions from wrecking too much havoc on the rail standard.
The 90 Degree Curve (90°きょくせんレ) was a mid-late 1970s set and single item first used in the 1974 EF15 Electric Locomotive Overpass Panorama Set. The individual release included two 90 degree curve rails and two orange track clips that could be used to hold a track joint together so the support piers can be placed elsewhere under the rails (for use in the case where an elevated joint is over another rail or other obstacle). The individual version was released in 1975 as number 39 under the 1970s numbering scheme and was out of print the following year. The plastic used to make these rails is a harder mix and 90 degree curves are thus more fragile than other contemporary rails. These rails were produced in Japan with roughtop tread.
90 Degree Curve Rails also appeared in the 1975 export Chug-A-Log Choo-Choo set with slightly different molding marks (two ringed pips in the "top" of the rail compared with three on the regular Plarail tooling). These were manufactured in the United States along with the straight rails in the set, the only time that Tomy produced Plarail rail in the USA.
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Around 2003 a new R-21 Double Curve Rail was released that serves the same purpose as the 90 Degree Curve Rail. It is produced from the same softer plastic mix as regular Plarail.