Single track propagated by G4 through ROOT geometry... current track ID=135 is an ionization electron created within the TPC.

At present, do not have the sensitive volumes initialized in the correct order...

*********************************************************************************************************
* G4Track Information:   Particle = e-,   Track ID = 135,   Parent ID = 12
*********************************************************************************************************

Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
    0 -1.56e+03     -542 -1.46e+03    0.0197        0        0         0        TPA1 initStep
    1 -1.56e+03     -543 -1.46e+03    0.0185  0.00127    0.584     0.584        TPA1 msc
    2 -1.56e+03     -543 -1.46e+03    0.0181 0.000368    0.372     0.956        TPSS Transportation
    3 -1.56e+03     -543 -1.46e+03    0.0176  0.00044    0.457      1.41        TPSS msc
    4 -1.56e+03     -543 -1.46e+03    0.0171 0.000535    0.457      1.87        TPSS msc
    5 -1.56e+03     -543 -1.46e+03    0.0166  0.00047     0.35      2.22        TPA1 Transportation
    6 -1.56e+03     -542 -1.46e+03    0.0162 0.000464    0.374      2.59        TPA1 msc
    7 -1.56e+03     -542 -1.46e+03    0.0154  0.00082    0.374      2.97        TPA1 msc
    8 -1.56e+03     -542 -1.46e+03    0.0152 0.000165    0.374      3.34        TPA1 msc
    9 -1.56e+03     -542 -1.46e+03    0.0147 0.000488    0.374      3.72        TPA1 msc
   10 -1.56e+03     -542 -1.46e+03    0.0141 0.000553    0.374      4.09        TPA1 msc
   11 -1.56e+03     -542 -1.46e+03    0.0136 0.000582    0.374      4.46        TPA1 msc
   12 -1.56e+03     -541 -1.46e+03    0.0129 0.000701    0.374      4.84        TPA1 msc
   13 -1.56e+03     -541 -1.46e+03    0.0101  0.00279    0.374      5.21        TPA1 msc
   14 -1.56e+03     -541 -1.46e+03   0.00795 1.97e-05   0.0378      5.25        TPA1 eIoni
   15 -1.56e+03     -541 -1.46e+03   0.00746 0.000495    0.374      5.62        TPA1 msc
   16 -1.56e+03     -541 -1.46e+03   0.00601  0.00144    0.374         6        TPA1 msc
   17 -1.56e+03     -541 -1.46e+03   0.00477  0.00125    0.374      6.37        TPA1 msc
   18 -1.56e+03     -541 -1.46e+03         0  0.00477    0.774      7.15        TPA1 eIoni

Found the magic location... maybe. Need to initialize immediately after TGeant4::Initialize is called. When I do this, at least I see that we are "doing something else" within a tracking volume...

*********************************************************************************************************
* G4Track Information:   Particle = proton,   Track ID = 1,   Parent ID = 0
*********************************************************************************************************

Step#    X(mm)    Y(mm)    Z(mm) KinE(MeV)  dE(MeV) StepLeng TrackLeng  NextVolume ProcName
    0    -1.13     2.04      202  1.94e+03        0        0         0        PVAC initStep
    1    -29.6    -25.4      202  1.94e+03 5.39e-08     39.5      39.5        PIPC Transportation
    2    -30.3    -26.1      202  1.94e+03    0.213        1      40.5        PWRP Transportation
    3    -30.5    -26.3      202  1.94e+03   0.0427    0.229      40.8        PIPE Transportation
    4    -30.5    -26.3      202  1.94e+03 1.25e-06   0.0789      40.8        CAVE Transportation
    5     -224     -212      201  1.94e+03   0.0326      268       309        CAVE hIoni
    :----- List of 2ndaries - #SpawnInStep=  1(Rest= 0,Along= 0,Post= 1), #SpawnTotal=  1 ---------------
    :      -224      -212       201    0.0235                 e-
    :----------------------------------------------------------------- EndOf2ndaries Info ---------------
    6     -330     -315      201  1.94e+03   0.0211      148       457        CAVE hIoni
    :----- List of 2ndaries - #SpawnInStep=  1(Rest= 0,Along= 0,Post= 1), #SpawnTotal=  2 ---------------
    :      -330      -315       201   0.00306                 e-
    :----------------------------------------------------------------- EndOf2ndaries Info ---------------
    7     -333     -318      201  1.94e+03  0.00107     4.51       462        TPCE Transportation
    8     -337     -322      201  1.94e+03 0.000327     4.93       467        TIFC Transportation
    9     -337     -322      201  1.94e+03  0.00499     0.02       467        TIKA Transportation
   10     -337     -322      201  1.94e+03   0.0319     0.15       467        TINX Transportation
   11     -346     -331      201  1.94e+03    0.128     12.7       479        TIAD Transportation
   12     -347     -331      201  1.94e+03    0.375      0.8       480        TIFC Transportation
   13     -347     -331      201  1.94e+03  0.00405     0.02       480        TPSS Transportation
   14     -433     -415      201  1.94e+03    0.019      120       601        TPAD Transportation
 StGeant4Maker:FATAL - TUnixSystem::DispatchSignals : segmentation violation Root > Function StartG4() busy flag cleared