Parallel Tracks on a Curve

[spokes32]

I'm curious as to the typical way parallel tracks are run through a curve.

For example, say you have a single track and you'd like to add a parallel track with 14'-0" track centers on tangent. On a 3 degree curve you'd like to increase track spacing to 14'-6" - how would you reach that 14'-6" clearance at the PC and PT of the curves? Would you increase the track centers in advance of the PC in order reach those required track centers?

[edbear]

I can't help too much, but a lot of early railroad lines were engineered for double track even though only a single track was initially constructed. It apparently is much easier to add a second track to a line engineered to a double track standard rather than remove a centered single track and then construct parallel double tracks. Maybe you can find a copy of RAILROAD CURVES AND EARTHWORK.

[JWilson]

I would either begin the spiral for the inner curve before beginning the outer curve spiral or, preferably, begin both spirals at the same point but reduce the rate of change on the outer spiral thus allowing the track centers to increase gradually and evenly until reaching the full body of the curve, where the centerlines will be the desired 14'-6" for your 3 degree curve.
Where the superelevation of the outer track is greater than that of the inner track, the center distance should be increased 1" for each 30 minutes of curvature, PLUS 3-1/2" for each 1" of difference in superelevation of the two tracks considered.