Where are we : On level ground or on a ramp - moving in a train? Let's say we are traveling in a train. The path has two parts: one at ground-level and the other moving up on the ramp. The ramp has an inclination of $\arctan\frac{a}{g}$ with the horizontal, where $a$ is the acceleration of the train on level ground and $g$ is the acceleration due to gravity.
The train does not accelerate on the ramp, but moves with a constant velocity.
Can we comment where we are (sitting inside the train of course!) when we have only a pendulum hanging on the roof to observe. (windows are blackened)

 A: If the train is an ideal isolated system, the answer is no, we can't tell where we are. As zhermes pointed out, the equivalence principle states that the gravitational force experienced  by a body at rest or moving with a constant velocity is the same as the pseudo-force experienced by that body is a non-inertial frame of reference. This simply states that the inertial mass is equal to the gravitational mass. To see that this is so, instead of thinking that the train is accelerating, imagine that you place another gravitational field that pulls on the pendulum in the same direction as the inertial force. Also, read about Einstein's thought experiment involving his famous elevator. In some ways its similar to your question.
A: Although the gravitational/inertial force causing the pendulum to tilt in the same way in both cases (see Equivalence principle link of @zhermes) thus not allowing you to see whether you are on accelerating or on the slope, you will probably be able to feel the difference.
The reason is that although the force parallel to the train is equal (which causes the pendulum to behave in the same way), the normal force acting on you is not so depending on the value of $\arctan\left(\frac{a}{g}\right)$ (whether it is sufficiently larger than 0) you will probably be able to feel whether you are on the incline or not.
[ADDITION after discussion with @markovchain]
Thinking along the line of parallel and normal forces I have to expand my answer a bit: a RIGID pendulum will look exactly the same on the flat and sloped part. A FLEXIBLE pendulum however, will be straight when moving up the slope, but curved when accelerated on the horizontal surface because it will have both a component pulling it to the left and a component (gravity) pulling it down.
