If the train is free people will choose it more often than driving or taking a uber/cab thus saving on CO2 since the train would've been running whether it's full or empty.
There are multiple aspects depending on what you're trying to accomplish.
Your list is great if the target market is upper middle class (great for voters and private interest groups). Convenient and plentiful are really the ones that matter for economic mobility (person that can't get afford car, gas,insurance)
“Nobody ever goes there anymore, it’s way too crowded!”
Really though, how is this an issue? A few people might not use trains anymore because they are packed full of new people? The net result is more people using trains
In Spain, public transportation already suffers from many problems. And this can only make it worse, because guess what? The government isn't spending a single cent in improving our network, it's only going to allow people to board for free. And I don't think anybody already using his car will now use the train, because despite being "free" now, it's not going to be better than using your own car. But you're right, I don't think anybody using the train will switch to the car again, because fortunately, this will only last until December. And it won't start until September, because the government didn't want the tourists to benefit from this measure.
