Transcription elongation is responsible for rapid synthesis of RNA chains of thousands of nucleotides in vivo. In contrast, a single round of transcription performed in vitro is frequently interrupted by pauses and arrests that drastically reduce the elongation rate and the yield of the full-length transcript. Here we demonstrate that most transcriptional delays disappear if more than one RNA polymerase (RNAP) molecule initiates from the same promoter. Anti-arrest and anti-pause effects of trailing RNAP are due to forward translocation of leading (backtracked) complexes. Such cooperation between RNAP molecules links the rate of elongation to the rate of initiation and explains why elongation is still fast and processive in vivo even without anti-arrest factors.