WordNet sense 1 (any sharply pointed projection):

spine, acantha

WordNet sense 2 (a verbalization that encourages you to attempt something):

goad, prod, goading, prodding, spurring, urging

WordNet sense 3 (a sharp prod fixed to a rider's heel and used to urge a horse onward):

gad

WordNet sense 4 (a railway line connected to a trunk line):

branch line, spur track