快速|かいそくkaisoku[0]heiban (flat)

1. no-adjectivehigh speed; rapidity
2. nounrapid-service train (not as fast as express); rapid train

• Synonym急速きゅうそくNeutralEmphasizes rapid change or development, as in 「急速に普及」; 「快速」 stresses speed itself and also names the JR rapid-service train.
• Synonym迅速じんそくNeutralStresses prompt, timely action, as in 「迅速な対応」; use it for quick handling or response.
• SynonymスピーディーNeutralLoanword for quick, efficient handling; colloquial, as in 「スピーディーな対応」.
• Synonym早いはやいNeutralThe everyday word for fast or early; the most common choice in daily use, and not specific to trains.
• Synonym速やかすみやかWrittenWritten word for promptness, usually as 「速やかに」, used in formal instructions.
• Synonym神速しんそくWrittenWritten word for speed so remarkable it seems superhuman, as in 「神速の攻撃」.
• Related手っ取り早いてっとりばやいNeutralColloquial for the quick-and-easy way that saves trouble; stresses convenience, as in 「手っ取り早い方法」.
• Related簡捷かんしょうWrittenRare written word for something quick and simple; stresses that the procedure itself is easy.
• Antonym鈍行どんこうNeutralColloquial term for a slow local train, the opposite of a 「快速」 train.
• Antonym低速ていそくNeutralLow speed; the direct opposite of 「快速」, stressing that the speed itself is slow.
• Antonym緩行かんこうWrittenRailway term for proceeding slowly, stopping at every station; written, the opposite of 「快速」.

Use 「快速」 for high speed or the JR rapid-service train, as in 「快速電車」 and 「快速に進む」. For prompt action say 「迅速」, for rapid change 「急速」, and in formal prose 「速やかに」. The main opposites are 「鈍行」 and 「低速」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. とても速はやいこと。
2. 快速列車かいそくれっしゃ。