The transit authority said Wednesday that it found and fixed the computer problem that led Metrorail to hold trains at stations last weekend. While service was relatively normal during this morning’s rush, the heat wave could slow trains later on as temperatures rise along aboveground tracks.

Metro said in a statement that a computer module about the size of a pizza box was the culprit this past weekend. When the module was replaced before train service began Wednesday morning, the errors in the train monitoring system disappeared.

On Saturday afternoon and again early Sunday morning, the system that allows controllers to see where the trains are failed. So the controllers ordered the trains to hold temporarily at the nearest stations.

The most likely cause of service issues for the rest of Wednesday is the heat wave.

Riders are likely to experience a repeat of the train slowdowns that occurred on all aboveground sections Tuesday. During our previous heat wave, a bend in the aboveground rails occurred on the Green Line. It’s the probable cause of the derailment that occurred on the afternoon of July 6.

Metro today is once again allowing riders to drink water on the trains and buses, as well as in the stations. That partly suspends the ban on eating and drinking within the transit system.

If Wednesday is like Tuesday, then bottles of water will come in very handy on Metrorail cars where the air conditioners either fail completely or can’t keep up with the high heat.

I took a temperature gauge with me Tuesday afternoon and rode cars on all the lines. Car temperatures ranged from 77 to 95 degrees. That high temperature was aboard car 5160 on the Orange Line, approaching Vienna. The low was aboard car 2069 on the Blue Line, near Pentagon station.

On days like this, you’ll have no doubt that you’ve entered a hot car. Use the intercom to tell the train operator, and move to another car at the next station. There can be quite a difference between adjacent cars.