Electrical means are the only way to achieve high speeds of a particle. Is it possible that you reach a limiting speed when energy radiates out as fast as it is put in? How is radiated energy accounted for in the total mass? There really is no non-electrical way to significantly accelerate a body other than ejecting a part of the mass, in which case the mass is not constant. I don't think a compressed spring or cannon will get you up near the speed of light. The speed a body reaches when it accelerates toward the earth from infinity is escape velocity, about 17000 mph. By the time you get near the speed of light by expelling mass you don't have any mass left.