Recently I visited Shanghai and took a ride in their MagLev train which tops at 431 kph. When it stopped accelerating and kept its constant speed, I was very surprised when I started walking up and down the aisle and felt strange, like I was walking uphill when moving toward the front -- even a bit unstable. This being a MagLev, there was absolutely no feeling of vibration or wobbling either. I remember feeling the same way during airplane flights but I attributed it to the fact that planes fly a bit tilted upward which in fact means I'm walking uphill as I move toward the front. But the train is completely level and I felt the same thing. I am 100% sure the train was not accelerating as I was near the driver and constantly observed the speedometer. If all objects inside the train are moving at the same speed as the train moving at constant speed and no other forces are exerted upon them as per Newton's 1st Law, I would assume that I would feel no different than being at zero speed. Then why would it feel harder and more unstable to move toward the front of the train?
How certain are you that it was level? Did you experiment on a return trip also? Maybe look at the altitudes of each station to find the average slope. If you get the chance take a helium balloon with you and it will show you the direction of the acelleration if there is any present
I suspect that constant velocity was not actually constant in all three dimensional axes. Even though forward speed might be close to constant, if there is any curvature in the track, deviation from rectilinear motion, the train and its contents can experience cetripetal accelerations.
Also if the train were moving even slightly uphill with any grade other than a straight ramp at constant forward speed there could be vertical accelerations away from the center of the earth creating an effective increase or decrease in gravity. If you were feeling 'heavier' that could account for your difficulty in moving.