The switch to the government's new measure of air pollution appears to be proving a challenge for the MTR.
Although the new Air Quality Health Index (AQHI) officially came into force at the end of last year, the Sunday Morning Post found the train company was still using the old Air Pollution Index (API) on Friday on its website and station display panels.
Following inquiries by the Post, however, the old index was removed - and now no figures are displayed.
The AQHI, widely used in the West, analyses the three-hour average concentration of four pollutants - ozone, nitrogen dioxide, sulphur dioxide and respirable suspended particles.
The new index uses a scale from 1 to 10+, with five levels of health risk, with 10+ denoting a "serious" risk to health.
The Environmental Protection Department said it had ceased to release API data on December 30.
The MTR said the information displayed in its stations was "obtained from a centralised system". For the API displayed on its website, the MTR said on Friday that it was generated through "programmed updating under a service arrangement".
"Changing information categories would require changes in service arrangements as well as system changes," it said. It was removing information from the displays in stations "to avoid confusion".
The department said it had advised the MTR to update its system and disseminate AQHI information as soon as possible.