The incident in Hang Chat district was the third derailment on the northern line in the past month and the ninth for the SRT this year. No one was injured.
The derailment took place on the same day that Transport Minister Chadchart Sittipunt travelled from Bangkok to Surin to get a first-hand look at conditions on the rail system.
Mr Chadchart earlier turned down an SRT request for a 10% increase in third-class fares, which have not risen in 20 years.
The incident in Lampang occurred around 7pm Friday when the left wheels of the 12-carriage train's sixth carriage went off the tracks, causing damages to more than 100 sleepers over a distance of 500 metres.
Around 300 passengers, 80% of them are foreigners, had to wait for three hours until the SRT could move everyone into the first five carriages to continue their journey north.
The SRT finally resumed normal service at 9.30am on Saturday.
The northern line was the scene of another derailment on Aug 2, when a northbound train went off the rails in a tunnel in Uttaradit.
Earlier, on July 13, the overnight express train derailed in Den Chai district of Phrae. The train was carrying 290 passengers, many of whom were foreign tourists. Thirty were injured, five of them seriously.
A total of nearly 900 accidents have occurred on the rail network nationwide in the past six years, with 297 deaths, including drivers and passengers in cars and trucks that collided with trains, often at poorly marked crossings.
The SRT has 100 billion baht in accumulated losses and has not done maintenance on some parts of its network for 30 years. Its rolling stock is in equally poor repair and the subject of frequent complaints about cleanliness.
The government has ambitious plans to overhaul the country's rail system as part of its 2-trillion-baht infrastructure programme. As part of the upgrade, the SRT would be asked to set up a separate agency to run a new high-speed rail business.
Mr Chadchart has conceded that the cost of running the third-class service was 10 times more than revenue the SRT raises from fares. The state agency lost about 10 billion baht last year.
However, he said the SRT had other options to increase revenue, especially by improving its services to draw more passengers.
Passenger traffic on the rail system has fallen by 40% since 1992 and freight traffic has dropped 30% since 2002, according to a report by the Asian Development Bank.
The minister spent 10 hours riding the rails on Friday, reaching his destination in Surin at 5.10pm, two hours behind schedule.
He observed that the section between Bangkok and Nakhon Ratchasima was slow but the train sped up after that.
He said the rail system still had problems because of single-track lines and old carriages as well as insufficient workers.
The SRT employs about 10,000 people, down from 30,000 earlier. A lot of its accumulated debt stems from pension obligations.
Mr Chadchart said he had gained valuable first-hand experience about complaints regarding the service, fighting and drinking on the train.
To improve overall service, he said that creating a double-track railway would be the first priority under the infrastructure budget.
Of the 2 trillion baht, 372 billion has been earmarked for the rail system upgrade and 753 billion for the high-speed network.