1. Boten–Vientiane Railway – Laos
The Boten – Vientiane railway line was officially opened on December 3, 2021 with a length of 422km, running between the capital Vientiane and the town of Boten on the border with Yunnan, China. Train lines here can operate at a maximum commercial speed of 160km/h, making the railway one of the fastest in Southeast Asia today.
2. KLIA Ekspres – Malaysia
KLIA Ekspres is a passenger transport service from Kuala Lumpur International Airport (KLIA). It runs from KL Sentral, the main railway station of Kuala Lumpur to KLIA as well as its low-cost terminal, klia2. The line can achieve a maximum commercial speed of 160km/h or a maximum speed of 200km/h without passengers.
3. Airport Rail Link (Bangkok) – Thailand
The Airport Rail Link (ARL) is an express and commuter rail line in Bangkok, Thailand. The line provides an airport rail link from Suvarnabhumi Airport, via Makkasan Station, to Phaya Thai station in central Bangkok. Most of the line is on a viaduct over the main eastern railway. The line is 28.6 km (17.8 mi) long and is elevated; the top speed is 160 km/h (99 mph), but the short distances between the stations do not permit commuter services to reach that speed.
4. KTM ETS (Gemas – Padang Besar) – Malaysia
The KTM ETS (commercially known as ETS, short for ‘Electric Train Service’) is an inter-city rail passenger service operated by Keretapi Tanah Melayu (KTM) using electric multiple-unit (EMU) trains. The KTM ETS is the second electric train service to be operated by the Malaysian railway company, with the ETS is the fastest metre-gauge train service in Malaysia. The ETS trains currently travel up to 140 km/h on the electrified metre gauge rail line.
5. Sprinter – Thailand
Sprinter is an air-conditioned diesel multiple unit of State Railway of Thailand. It was manufactured by BREL in Great Britain. It is also the first air-conditioned diesel multiple unit with driving cab in Thailand. There were initial 12 cars with driving cab (2501-2512) and 8 cars without driving (2113-2120). Its max speed was 145 km/h in England, but changed to 120 km/h in Thailand.
According to Wikipedia, futuresoutheastasia.com AND thai-railway.fandom.com