Here is a scenario in which two twins age at different rates even though both always have the same speed. Consider a train track that includes a circular segment with ground circumference minutely greater than 80. Two twins are on a train with synchronized watches (Frank at the front and Reba at the rear). The train has proper length 100 and velocity in the inertial ground frame of 0.6c. While traveling inertially (point 1 in the diagram below), in the ground frame the train’s length is 80 and Reba’s watch is ahead of Frank’s by the product of the train’s proper length and its speed, or 60. The tracks shunt the train into the circular segment as it passes. After completing the circuit around the circle, the front of the train exits just as the rear has entered, and Frank grazes Reba (point 2 in the diagram below). At this event, they compare watches. Each has traveled at the same speed the entire time (0.6c), and therefore Reba’s watch remains 60 ahead of Frank’s in the ground frame. Because they are both at the same event, Reba is 60 older than Frank. Frank has aged 60 less than Reba during his circuit. Thereafter Frank and the front of the train travel inertially to the right; Reba and the rear of the train complete the circuit, exit the circle, and then continue inertially to the right (point 3 in the diagram). Each watch has continued to travel at the same speed; Reba’s watch continues to be 60 ahead of Frank’s in the ground frame; but their watches are now synchronized again in their inertial frame. Reba has aged 60 less than Frank during her circuit. In this scenario the twins age at different rates during their travels even though they always travel at the same speed. The change of direction acceleration within the circle causes the different aging. Speed has an impact, but only on the amount of differential aging during acceleration (the product of the train’s proper length and speed). The proper length has an important role also. If you double the proper length of the train and the ground circumference of the circle while maintaining the train’s speed at 0.6c, you double the time during which the train undergoes change of direction acceleration, and you double the amount of differential aging that occurs during the acceleration – even though the twins continue to travel at the same speed at all times, and it is the same speed (0.6c) as they traveled on the shorter circumference. I’d appreciate your general thoughts on the scenario and any specific views you can offer on which twin’s worldline is longer during the period between Frank entering the circle and his grazing Reba. Presumably his worldline is a helix and hers is a straight line; each travels the same distance in the ground frame over the same time in the ground frame; but I do not know how one measures each worldline to determine which is longer, and what the measure of each is. Thanks.