Standing at stud at the Big Red Farm in Hokkaido, he produced numerous important winners including Gold Ship, Orfevre, and Nakayama Festa.

[8] On his final start of the year Rainbow Line was dropped back in distance for the Sanryo Sho over 1600 metres at Hanshin Racecourse on 27 December and won narrowly from Sunrise Couronne.

[7] Two weeks after his last start of 2015, Rainbow Line began his second campaign in the Shinzan Kinen at Kyoto and finished sixth of the eighteen runners behind Logi Cry.

On 27 February at Hanshin the colt contested the Grade 3 Arlington Cup over 1600 metres on firm ground and started the 5.8/1 fourth choice in the betting behind Urban Kid, Ball Lightning (Keio Hai Nisai Stakes) and Hiruno Magellan.

After a break of two and a half months Rainbow Line returned to the track in August when he was matched against older horses for the first time in the Grade 2 Sapporo Kinen.

After racing towards the rear for most of the way he produced an "impressive late charge" to take second place behind Satono Diamond with Air Spinel and Dee Majesty finishing close behind in third and fourth.

[7] After a break of more than five months Rainbow Line returned in the autumn edition of the Tenno Sho in which he started a 58.6/1 outsider and produced a strong late run to take third place behind Kitasan Black and Satono Crown.

He started the 4.6/1 third choice in the betting behind Clincher (Kyoto Kinen) and Albert (Stayers Stakes) in an eleven-runner field which also included Satono Chronicle (Challenge Cup) and Curren Mirotic (Kinko Sho).

Cheval Grand gained the advantage in the straight but Rainbow Line threaded his way through field after being switched to the inside by Iwata, caught the favourite in the final strides and won by a neck.

[16] Immediately after the race the horse was found to be lame, having sustained an injury to his right foreleg and was taken receive treatment at a veterinary facility rather than taking part in the victory ceremony.

Iwata said "While our rivals were making early moves in the backstretch, my mount waited patiently, and I knew he had the needed late kick to reach the front before the wire.