Okay, let's do the math. Consider a 2 hour flight, say Madrid -> Paris. A round trip is something like 140 eur. Boeing 737-800 burns something like 3000 liters of jet fuel/hour, and will typically carry something like 150 passengers. That's 40 liters per person. Jet fuel is really pricey now, something like 80 cents/liter. This means that fuel costs something like 30 eur per flight per passenger, so 60 eur in a round trip.
Now imagine we slap a tax on jet fuel similar to the one on vehicle gasoline. This would mean that the price of jet fuel would go up by 50%. The round trip ticket Madrid - Paris now costs 170 eur instead of 140 eur. Would it make Madrid - Paris train route competitive? No, not at all.
Truth is, you can make anything competitive with anything else if you put heavy enough finger on the scale, but the reason flights are much cheaper than trains, despite being faster, is not because of lack of jet fuel taxes.
Now imagine we slap a tax on jet fuel similar to the one on vehicle gasoline. This would mean that the price of jet fuel would go up by 50%. The round trip ticket Madrid - Paris now costs 170 eur instead of 140 eur. Would it make Madrid - Paris train route competitive? No, not at all.
Truth is, you can make anything competitive with anything else if you put heavy enough finger on the scale, but the reason flights are much cheaper than trains, despite being faster, is not because of lack of jet fuel taxes.