def railway_ST(C, start, end, railway_r):
delta = atan2(*(end - C)[0:2]) - atan2(*(start - C)[0:2])
dbg('railway_ST C', C, 'start', start, 'end', end, 'delta', delta)
+ if delta < 0: delta += tau
s = delta * railway_r
dbg('railway_ST delta', delta, 'r', railway_r, 's', s)
return s