the question would be , can you accelerate a train to the speed off light ? if you remove the speed of light out of that hipothesis , any speed you are moving at , if you move on the same axis at X speed , you will add the X speed to the speed you are moving at .... introducing the speed of light brings extra problems ... can a particle with mass even reach the speed of light . i dont think so , i believe you would need infinite amounts of energy which would make it impossible .

lights speed is absolute , everything else is relative ....

if are standing still you shine a light at a wall , the light will travel at 299 792 458 m / s towards the wall .... if you happen to be traveling in a bus towards a wall at 100 km/h , and you shine the light forward , it will still travel at 299 792 458 m / s .. lights speed is absolute ....

however if you instead of shinning a light , you shoot a bullet out a gun , standing still , the bullet will travel towards the wall lets say at 300 meter per second ... but if you are in a bus traveling at 100 km/h and you shoot the guns you will have to add up the speeds to get the bullet speed ....