  At the SASE-FEL user facilities FLASH and European XFEL, superconducting
TESLA type cavities are used for acceleration of the driving electron bunches.
The high achievable duty cycle allows for operating with long bunch trains,
hence considerably increasing the efficiency of the machine. However,
multibunch free electron lasers (FEL) operation requires longitudinal and
transverse stability within the bunch train. The purpose of this work is to
investigate the intra-bunch-train transverse dynamics at FLASH and European
XFEL. Key relationships of superconducting rf cavity operation and the
resulting impact on the intrabunch-train trajectory variation are described.
The observed trajectory variation during multibunch user runs at FLASH is
analyzed and related to both, intrabunch-train variations of the rf and the
following impact on the multibunch FEL performance.
