Liking trains is not inherently a sign of autism, though trains are a common and well-documented special interest among autistic people. Special interests—deep, focused enthusiasm for specific topics—are a recognized characteristic of autism spectrum disorder, and trains are frequently cited examples because they combine visual appeal, mechanical complexity, predictability, and rule-based systems that can be intrinsically rewarding to autistic minds.
However, many non-autistic people enjoy trains for similar reasons: the engineering, history, photography opportunities, or nostalgic appeal. Train enthusiasm exists across the general population, including railroad engineers, historians, hobbyists, and casual fans.
What might suggest autism is not the interest itself, but the pattern of engagement: whether the interest is unusually intense, narrow, or all-consuming compared to age-typical peers; whether it involves repetitive behaviors related to the topic; or whether it coexists with other autistic traits like sensory sensitivities, social communication differences, or preference for predictability. A child who loves trains as one of several hobbies differs from one for whom trains dominate daily thoughts, conversations, and play in ways that interfere with other activities.
Autism is diagnosed by overall developmental and behavioral patterns, not by any single interest. If you're concerned about autism in yourself or someone else, the relevant factors include social communication, repetitive behaviors, sensory experiences, and how these affect functioning—not the specific topic of interest. A professional evaluation is the only way to determine whether autism is present.