You appear to be confusing momentum and energy.
Both are conserved. But momentum is a vector quantity, and energy is not.
You can only change either by interacting with something else.
A space ship traditionally changes its momentum by firing rockets (splitting itself into payload and propellant), or by interacting with other objects (light from a sun, or gravity from a heavy body).
A space ship in orbit around another body which does an impulse burn of thrust will actually return back to its starting location, unless it generates enough thrust to escape the gravity well of the thing it is orbiting. In a more complex situation it may not (as it interacts with more bodies).
For the marble, it is being turned through a different mechanism. It is interacting with the track, which is in turn anchored to a planet whose mass is much much larger than the marble. 343
If the track is rigid, and marble is smooth, and the corners are rounded, the marble will change direction without changing velocity. Momentum will be exchanged between the marble and planet (the planet, being very large, will be changed by a degree nobody could detect), and the marble will change direction.
Under the assumptions of a rigid connection and zero friction, very little to no energy will be lost by the marble.
A space ship with frictionless wheels on it that rode a ramp that was much, much more massive than the space ship would also be able to do a 180 degree turn without losing much energy.
You may have heard of the terms "elastic collision" and "inelastic collision". An elastic collision is one where next to no energy is lost in the deformation of the colliding bodies or sound of impact.
You can model a marble track as an elastic collision.
When you solve for the result of an elastic collision between a small object and a heavy object, you'll find that almost no momentum and energy is transmitted from the small object to the heavy object.
If you solve the equations of elastic collision ($m$ is mass, $u$ is initial velocity, $v$ is end velocity), with $m_2 \gg m_1$ and $u_2 = 0$ (massive object starts out stationary),
$$v_1 = u_1 \frac{m_1-m_2}{m_1+m_2} \approx u_1$$ $$v_2 = u_1 \frac{2m_1}{m_1-m_2} \approx 0$$
the large object doesn't budge noticeably, and the small object reflects.
Energy is conserved, momentum is reversed on the small object, and the large object ends up moving at a ridiculously slow speed.
Because the energy of an object is proportional to the square of the velocity, that ridiculously slow speed results in an even ridiculously smaller change in kinetic energy, leaving almost all of it for the light object to maintain its velocity.
In the case of a track firmly connected to the Earth, the 6 gram marble is $10^27$ times lighter than the planet. This means in the "ideal" situation, as little as 1 part in $10^27$ of its momentum is transferred to the planet.
In practice, the transfer of momentum from the ball to the planet won't be perfect, but you can get close enough that rolling friction and wind resistance is going to dominate over that problem. 343