The collision occurred at 10:15pm yesterday evening in Ayutthaya province when a rapid train from Bangkok to Denchai slammed into the back of a stationary express train.
"Of the 52 injured, six people remain in hospital, two are in critical condition," Thanongsak Kongprasert, a senior official at the State Railway of Thailand, told AFP.
He added that six carriages had fallen off the tracks in the crash and that the two victims fighting for their lives in hospital were the driver and technician on board the Bangkok-Denchai train.
Local disaster planning official Udonsak Kawnuna told AFP that two foreigners, an American and South Korean, were injured and under observation at a nearby hospital.
Despite being one of Southeast Asia's wealthier nations, Thailand has a decrepit and slow railway system that the military junta is keen to overhaul.
The country has just 250 kilometres (155 miles) of dual track railway, making train travel painfully slow and collisions are not uncommon.
The government plans to build a series of new lines over the coming years, the first of which will be built with Chinese expertise.