Final answer:
The force constant of the spring is 1.25 x 10^5 N/m.
Step-by-step explanation:
To find the force constant of the spring, we can use the equation:
k = (1/2)mv^2 / x^2
where k is the force constant, m is the mass of the subway train, v is the initial velocity, and x is the distance over which the train is brought to a stop.
Plugging in the values given in the question, we get:
k = (1/2)(5.00 x 10^5 kg)(0.500 m/s)^2 / (0.400 m)^2 = 1.25 x 10^5 N/m.
Therefore, the force constant of the spring is 1.25 x 10^5 N/m.