LONDON — Sarah de Lagarde was rushing to a train in September 2022 when she slipped and fell through a gap between the platform and the train. For 15 horrifying minutes, she was stuck on the tracks undetected. Two trains ran over her. She survived, but her right arm and the lower portion of her right leg had to be amputated. Laying in a hospital bed after multiple surgeries, de Lagarde, who just a month earlier had hiked Mount Kilimanjaro with her husband, Jeremy, wondered what the rest of her life would be like. “I had thought I was invincible,” de Lagarde, a public relations executive at an investment firm in London, said in an interview. She began thinking about what she could do.