Express Train is among the first five foals out of the lightly raced I'm a Flake (Mineshaft). He is a full brother to a winner and a half-brother to Praetorian (Tiznow), who won six races. I'm a Flake is a half-sister to seven winners, headed by the Group 2-placed November Slew (Seattle Slew), dam of Hiraboku Wild, a Listed winner in Japan, and grandam of Ezmosh, a Group 3 winner in the US.

His second dam is November Snow (Storm Cat), a multiple Grade 1 winner in the US whose wins included the Grade 1 Test Stakes and the Grade 1 Alabama Stakes. Besides Express Train she is also the grandam of Morning Line (Tiznow), a sire who won the Carter Handicap, then a Grade 1, of the sire Kuroshio, a dual Group winner in Australia, and of his half-sister Believe'n'Succeed, a Group 3 winner. Believe'n'Succeed is the dam of Anthony Van Dyck, winner of the Gr.1 Derby, and of Bounding, a dual Group 1 winner in Australia.

The third dam Princess Alydar (Alydar) was placed and has produced 11 winners from 13 foals, among them the Group 3 winner Scatmandu (Storm Cat) and the stakes winner Lady Sorolla (Pancho Villa).

I'm a Flake was barren for two years after Express Train and then produced colts in 2020 and 2021: Union Express (Accelerate) and one by Union Rags. Munnings covered her in the spring of 2022.