Tokyo's station serves 3000 trains daily and some 415K passengers per day as of 2013.
The Chinese high-speed rail system also deals with similar levels of passenger traffic. Shanghai's Hongqiao station for instance has no less than 30 tracks, can hold 10K passengers at the same time, and comfortably serves an average of 210K passengers per day (much more on holidays), and it is only one of a few railway stations serving the area.
Trains scale well if you do them right. Technology has come a long way.
That's what life would look like if people in the US actually used trains on a regular basis.
In any case, all I'm saying is that if a massive number of people need to be moved, trains can scale well, additional reserve capacity can be deployed during evacuations, and it's also a lot safer than having hundreds of thousands of people taking to the roads in bad weather.
Tokyo is crammed and that's why trains work great over there. They wouldn't work in US city with suburban sprawl.
But let's take your numbers: 415K passengers per day. Let's keep the same three day evacuation period. That's ~1.2 million people. Leaving 5 million in Houston.
Here's the point: Evacuating millions of people over a weekend is a gargantuan task. Magic words like "train", "plane", or "automobile" don't instantly solve the problem.