Three weeks before Singapore's largest running event next month, organisers told participants that, unlike previous years, they would not be able to take the MRT to the Standard Chartered Marathon Singapore (SCMS).

SCMS said on Facebook on Nov 14 that it was informed only "recently" of this development, despite discussions with the authorities that started months earlier.

However, SMRT said it gave ample notice that it would not be able to provide train services due to rail engineering works on Dec 4, the day of the race. It had informed organisers of its decision in May, it said.

"The organiser was given ample notice to plan alternative travel arrangements and communicate these early to its participants," said Mr Patrick Nathan, SMRT's vice-president for corporate information and communications.

He added that SMRT is reaching a "critical point" in its network maintenance works, which include "sleeper replacement, re-signalling and third-rail replacement efforts".

Such projects are planned as far as a year in advance, he added.

He pointed out that "every minute of track access time is valuable" because engineering staff have only around three hours every night to work.

Since 2011, SMRT has been starting train services earlier on the day of the marathon, to help runners make it to the starting point in time. This year, the marathon will flag off as early as 4.30am.

The SCMS is being held for the 15th time this year, and there are 52,000 spots across five categories.

Organiser Ironman Endurance Asia said it has arranged for buses to ferry participants and is also working with taxi companies to take them to the race site.The firm declined to say exactly when it was informed by SMRT that train services could not be provided.

Marathon runners argued that train services should be provided for this year's race, since the half- and full-marathons flag off at the same time, at a common starting point in Orchard Road.

Said Mr Kang Yew Khim, 46, an accountant who is taking part this year: "You are talking about tens of thousands of people. With the road closures, and buses driving into the area, there will definitely be some bottlenecks. You can't afford to be late for a marathon."