I've done 40+ overnight train and bus journeys across different continents and I want to share the actual hierarchy of options by comfort and cost.

Top tier — Southeast Asian overnight sleeper buses: Vietnam in particular has good-quality 'sleeper bus' services where you get a fully reclining pod, not just a reclining seat. The Hanoi to Hoi An overnight run (~14 hours) costs about $20-25 and the better operators have clean buses with reasonable leg room even for taller travelers. Book through a local agency or directly at the bus station, not through hostel commission.

Solid middle tier — European overnight trains with couchettes: The Vienna to Krakow EuroNight, Paris to Madrid, Zurich to Barcelona all still run. Couchette (shared sleeping compartment with 4-6 berths) costs €30-60 on top of the base ticket. Saves a night of accommodation and gets you there without airport time. Book early for the cheap beds.

Variable quality — Indian sleeper trains: Second-class sleeper (SL) in India is extremely cheap ($5-10 for long overnight routes) but comfort varies. The 3-tier air-conditioned (3A) class is a step up at $15-25 and worth it on longer journeys. Book on IRCTC at least 60 days out; popular routes sell out.

Where buses beat trains: almost everywhere in Central America and the Balkans where rail infrastructure is minimal. Shuttle buses between tourist hubs are marginally more expensive than local buses but dramatically safer and more comfortable at night.