Metro-North says trains are expected to be more crowded Wednesday morning because of fewer cars on the New Haven Line.

It says the reason is “due to the unavailability of several train cars requiring wheel repair on account of yesterday's weather related slippery rail conditions.”

The railroad sent out the alert around 5:30 a.m. Wednesday.

At this point, Metro-North says the crowded trains should only affect morning railroad passengers.

The “slippery rail conditions” were caused by the combination of rain and fallen leaves.

The leaves that are crushed on the track are made into a gelatinous, slime-like substance that reduces the normal amount of adhesion train wheels have on the rails.

This “slip slide,” prevents trains from stopping normally when engineers apply the brakes.

In severe cases the train will automatically make an emergency stop, because the on-board computer system perceives slip-sliding as the train not slowing down when it should. And this slip-sliding during braking also creates flat spots on the train's wheels

According the a rail engineer web site, “Rapid transition from slippery rail to dry rail and back can sometimes result in flats being created on wheels during braking. The flat spot occurs when a rail vehicle’s wheelset is dragged along the rail after the wheel/axle has stopped rotating. Flat spots are usually caused by use of the emergency brake, or slip and slide conditions that cause wheels to lock up while the train is still moving.”

Metro-North needs to remove the slime and check the train wheels for any damage.

Crews will also spray water at high pressure and use rail scrubbers to remove the crushed leaf goo from the tracks.

Buy this photo Photo: Christian Abraham / Christian Abraham

The rain on Tuesday and early Wednesday brought down many of the remaining leaves.

More leaves also fell because of strong wind. The strongest wind gusts - up to 21 mph happened - between 5 and 8 p.m. Tuesday, according to data from the National Weather Service.

During that time Tuesday night, Metro-North had delays of more than 20 minutes because of slip-slide conditions.