OTP stands for On-Time Performance, in train lingo. It refers to the number of times that a given train, or railway company, had late trains.
The OTP is usually given in a percentage value, and refers to the percentage of trains that were "on time". Whether a train is on time or not depends on how long the route is. A train from Toronto to New York City has about 10 minutes leeway, but a train from New York to Florida would have to be about an hour and a half late before it was officially considered "late" in the books.