In recent years, we've become increasingly aware of how vulnerable trains and subways are to bomb attacks.
In 2004, there were the Madrid train bombings which killed 191 people and injured 1,800.
A year later, there were the London train bombings that killed 52 people and injured more than 700.
Well, since then, a team of rail engineers in Britain have been looking at ways to reduce the number of casualties in those kind of attacks.
And today, they unveiled their idea: a "bomb-proof" train.
Now, to be clear, it's not entirely "bomb-proof". But their design definitely seems to hold up a lot better during an explosion.
The engineers, from Newcastle University, found that a cheap plastic coating on the train's windows can help prevent glass from shattering onto bystanders.
They also use lighter panels that absorb energy, which could reduce the impact of a bomb and the amount of flying debris.
Conor O'Neill is the chief engineer from Newcastle University's NewRail research centre.
"What we've shown is that companies could make some relatively cost-effective and simple modifications that would significantly improve the outcome of an attack," he told Sky News.
With high-speed cameras, the engineers filmed an explosion on a regular train and analyzed what happened as soon as the bomb went off and how the blast travelled through the car.
From there, they built a prototype designed to be "blast-resistant."
Here's a video that shows explosions on both types of train.
In the first two shots, with the regular train, the windows and doors get blown out and the roof gets blown off.
When you compare that to the modified train, around the :33 mark, only the safety windows get blown out - which are designed to do that.
The other windows, the doors, and the roof stay intact - so the explosion is contained.
"Preventing flying objects is the key," said O'Neill.
"Tethering ceiling panels reduced the risk of fatalities and injury from flying shrapnel and meant the gangways were kept relatively clear of debris, allowing emergency staff quick access to the injured."
"The window coating we developed was also incredibly effective. Without it the windows are blown outwards - putting anyone outside, such as those standing on a platform, at risk from flying glass," he said.
O'Neill made it clear a bomb on a train is always going to be devastating.
"We have to be realistic - completely replacing existing vehicles just isn't an option," he added.
But by developing new technology and materials for existing trains, it could make a significant difference.
"These are all low-cost, simple solutions that can be put on existing trains which could not only save lives," he said, "but also reduce the attractiveness of our railways for potential terrorist attacks."