Measurements of the total drag force on a stationary, solid model of a Taylor bubble placed in a downward flowing liquid stream in a vertical tube have been performed and analyzed using a one-dimensional flow model, in order to study the mechanisms responsible for acceleration of a Taylor bubble in the wake region of a preceding Taylor bubble in gas-liquid slug flow. Variations in the drag force with the solid Taylor bubble's lateral displacement from the tube axis were determined in single and dual bubble systems. The single bubble experiments with 7.5- and 15-cm long, bullet shaped solid bubbles with axi-symmetric and deformed nose shapes showed a significant decrease in the drag force with Taylor bubble displacement from the tube axis for all bubbles. For the deformed nose bubbles, the measured drag force was significantly smaller than for the normal nose bubbles at all lateral bubble positions. In the dual bubble system, the drag force measured on a trailing bubble at the center of the pipe increased gradually with the increasing separation distance up to about two pipe diameters, while beyond this region the drag force remained essentially constant. A one-dimensional flow model was developed and used to determine the factors responsible for drag force reduction. Based on these results, acceleration of a trailing Taylor bubble in a gas-liquid slug flow has been attributed to the reduction in drag force caused by changes in the nose shape and lateral displacement of the bubble from the vertical pipe axis.