A Railways spokesperson said around 18 trains are running late by one-and-a-half to five hours.
Operations at the Delhi airport remained normal, an official said.
On Tuesday night, three flights were returned or diverted to the Delhi airport due to bad weather in Chandigarh, Varanasi and Lucknow, he said.
The Palam airport logged the lowest visibility level of 400 metres at 2:30 am. Visibility dropped to 500 metres at the Safdarjung airport at 5:30 am, the India Meteorological Department (IMD) said.
According to the IMD, very dense fog is when visibility is between 0 and 50 metres, between 51 and 200 metres is dense fog, 201 and 500 metres is moderate fog, and 501 and 1,000 is shallow fog.
The Safdarjung Observatory, the primary weather station in Delhi, recorded a minimum temperature of 7.1 degrees Celsius, a notch below normal. The maximum temperature is likely to settle around 22 degrees Celsius, the IMD said.
The minimum and maximum temperatures are likely to drop to 5 degrees Celsius and 20 degrees Celsius in the next few days, it said.