脱線

1. Origins, structure, and basic meaning

• Structure: 脱線 (だっせん) can mean a train leaving the rails or a conversation leaving its subject.
• Basic usage: A film depicts a rain-caused derailment; a discussion goes off track and returns to the main point.

2. Common pitfalls and similar expressions

• Rail accident and figurative conversational digression
  • First subject 列車が creates a physical accident; second 話が is discourse metaphor. The same past form spans different domains.