A, when learning acrobats' tightrope performance, we should look at the actors' movements, and we can't regard acrobats as particles, so A is wrong;

B, when studying the posture of athletes using pole vault, it depends on the athletes' movements and cannot be regarded as particles, so B is wrong;

C. When observing the take-off attitude of the carrier aircraft on the aircraft carrier, the size of the aircraft carrier cannot be ignored and cannot be regarded as a particle, so c is wrong;

D, when studying the running speed of the train from Shunde to Zhuhai, the size of the train can be ignored and can be regarded as a particle; Therefore, d is correct;

Therefore, choose: d.