This actually isn't a secret... just not real well known. First of all you must know all the PI coordinates and all the spiral and curve lengths. An input file can then be created, loaded and read using COGO. An example of this type of input file is shown below:
STORE POINT 201 493808.91 2646981.59 STATION 373+25.94 STORE POINT 202 493399.44 2649143.47 STORE POINT 203 492197.22 2648735.10 STORE POINT 204 478828.80 2649139.35 STORE POINT 205 478908.75 2651839.15 STORE POINT 206 473605.02 2651991.02 STORE POINT 207 473703.52 2654645.70 STORE POINT 208 469889.89 2654736.44 STORE POINT 209 467112.31 2654753.47 STORE POINT 210 465216.02 2654800.36 STORE POINT 211 463099.85 2654886.58 STORE POINT 212 463115.07 2655513.30 STO SCS A PB 201 PI 202 LS1 200 LS2 200 R 716.20 PA 203 STO SCS B PB 202 PI 203 LS1 150 LS2 150 R 954.93 PA 204 STO SCS C PB 203 PI 204 LS1 300 LS2 300 R 954.93 PA 205 STO SCS D PB 204 PI 205 LS1 450 LS2 450 R 954.93 PA 206 STO SCS E PB 205 PI 206 LS1 450 LS2 450 R 954.93 PA 207 STO SCS F PB 206 PI 207 LS1 320 LS2 320 R 716.20 PA 208 STO SCS G PB 207 PI 208 LS1 250 LS2 250 R 22918.31 PA 209 STO SCS H PB 208 PI 209 LS1 300 LS2 300 R 19098.59 PA 210 STO SCS I PB 209 PI 210 LS1 250 LS2 250 R 22918.31 PA 211 STO SCS J PB 210 PI 211 LS1 300 LS2 300 R 477.46 PA 212 STO CHA AAA 201 SPI AB CUR A SPI AA SPI BB CUR B SPI BA STO CHA BBB SPI CB CUR C SPI CA SPI DB CUR D SPI DA STO CHA CCC SPI EB CUR E SPI EA SPI FB CUR F SPI FA STO CHA DDD SPI GB CUR G SPI GA SPI HB CUR H SPI HA STO CHA EEE SPI IB CUR I SPI IA SPI JB CUR J SPI JA 212 STO CHA ANT CHA AAA CHA BBB CHA CCC CHA DDD CHA EEE STA 373+25.94NOTE: As with all COGO input files... there must be a blank line here at the bottom or the last line will NOT be read.