a   Parameter in Davis equation (N)
aB   Ballast acceleration (m/s2)
aT   Train deceleration (m/s2)
A   Reference area of train (m2)
AB   Ballast area (m2)
Aeq   Equivalent leakage area (m2)
ATU   Tunnel area (m2)
b   Parameter in Davis equation (Nsm1)
b1   Parameter in Davis equation (Nsm1)
b2   Parameter in Davis equation (Nsm1)
B   Train cross-sectional area to tunnel cross-sectional blockage ratio
c   Parameter in Davis equation (Ns2m2)
ca   Speed of sound in air (ms1)
CD   Drag coefficient
CDB   Ballast drag coefficient
CDBO   Drag coefficient of bogies
CDNT   Drag coefficient of train nose and tail
CDW   Contact wire drag coefficient
CD(ψ)   Drag coefficient at yaw angle ψ
CDf(ψ)   Drag coefficient increment at yaw angle ψ due to increased wheel/rail ...

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