We've searched the internet and found the largest collection of On30 Scale trains for sale is on eBay. Below are the listings for 'boxcar'.

RUDOLPHS  ABOMINABLE SNOW MONSTER CHRISTMAS TOWN EXPRESS BOX CAR
RUDOLPHS ABOMINABLE SNOW MONSTER CHRISTMAS TOWN EXPRESS BOX CAR
$74.99
Time Remaining: 2d 47m
Buy It Now for only: $74.99
Sandy River Car Shops O scale B8 Phillips boxcar storage shed WOOD kit
Sandy River Car Shops O scale B8 Phillips boxcar storage shed WOOD kit
$26.95
Time Remaining: 4d 3h 38m
Buy It Now for only: $35.50
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
$37.00
Time Remaining: 5d 4h 30m
Buy It Now for only: $37.00
Bachmann On30 Scale Train Box Car Murphy Roof Oxide Red 2 box 26552
Bachmann On30 Scale Train Box Car Murphy Roof Oxide Red 2 box 26552
$89.55
Time Remaining: 12d 7h 33m
Buy It Now for only: $89.55
Bachmann On30 Scale Train 18 Freight Cars Box Car 2 per box 26501
Bachmann On30 Scale Train 18 Freight Cars Box Car 2 per box 26501
$89.55
Time Remaining: 12d 7h 33m
Buy It Now for only: $89.55
Bachmann On30 Scale Train 18 Box Car W Murphy Roof Gray 2 box 26553
Bachmann On30 Scale Train 18 Box Car W Murphy Roof Gray 2 box 26553
$89.55
Time Remaining: 12d 7h 33m
Buy It Now for only: $89.55
Bachmann On30 Scale Train Box Car South Pacific Coast Sunset Logo 27002
Bachmann On30 Scale Train Box Car South Pacific Coast Sunset Logo 27002
$44.40
Time Remaining: 12d 7h 35m
Buy It Now for only: $44.40
Bachmann On30 Scale Train Box Car Mineral Red Data Only 27098
Bachmann On30 Scale Train Box Car Mineral Red Data Only 27098
$44.40
Time Remaining: 12d 7h 35m
Buy It Now for only: $44.40
Bachmann On30 Scale Train Box Car Oxide Red Data Only 27097
Bachmann On30 Scale Train Box Car Oxide Red Data Only 27097
$44.40
Time Remaining: 12d 7h 35m
Buy It Now for only: $44.40
Bachmann On30 Scale Train Box Car White Pass  Yukon 27001
Bachmann On30 Scale Train Box Car White Pass Yukon 27001
$44.40
Time Remaining: 12d 7h 35m
Buy It Now for only: $44.40
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 15 nd
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 15 nd
$66.07
Time Remaining: 14d 18h 37m
Buy It Now for only: $66.07
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
$66.07
Time Remaining: 15d 18h 42m
Buy It Now for only: $66.07
HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR J E B STUART BOX CAR 005
HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR J E B STUART BOX CAR 005
$59.99
Time Remaining: 16d 10h 29m
Buy It Now for only: $59.99
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 605
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 605
$66.07
Time Remaining: 21d 21h 54m
Buy It Now for only: $66.07
2 On30 Spectrum Colorado  Southern boxcars NIB
2 On30 Spectrum Colorado Southern boxcars NIB
$59.99
Time Remaining: 25d 14h 43m
Buy It Now for only: $59.99
On30 Spectrum Dismal Swamp boxcar NIB
On30 Spectrum Dismal Swamp boxcar NIB
$35.00
Time Remaining: 25d 14h 44m
Buy It Now for only: $35.00
On30 Spectrum South Pacific Coast boxcar NIB
On30 Spectrum South Pacific Coast boxcar NIB
$29.95
Time Remaining: 25d 14h 44m
Buy It Now for only: $29.95
On30 Spectrum Ohio River  Western boxcar NIB
On30 Spectrum Ohio River Western boxcar NIB
$31.00
Time Remaining: 25d 14h 44m
Buy It Now for only: $31.00
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 606
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 606
$66.07
Time Remaining: 27d 18m
Buy It Now for only: $66.07
On30 On3 Pro built Wood Boxcar Model
On30 On3 Pro built Wood Boxcar Model
$119.95
Time Remaining: 27d 16h 32m
Buy It Now for only: $119.95
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
$65.80
Time Remaining: 28d 2h 59m
Buy It Now for only: $65.80

Other On30 Scale train items recently purchased through this site:

On30 Fat Albert Boxcar Conversion to fit Bachmann 18ft flat car
On30 Spectrum South Pacific Coast boxcar NIB
On30 Spectrum Dismal Swamp boxcar NIB
On30 Spectrum Ohio River Western boxcar NIB
Sandy River Car Shops O scale B8 Phillips boxcar storage shed WOOD kit
2 On30 Spectrum Colorado Southern boxcars NIB
RUDOLPHS ABOMINABLE SNOW MONSTER CHRISTMAS TOWN EXPRESS BOX CAR
Bachmann On30 Scale Train Box Car Oxide Red Data Only 27097
Bachmann On30 Scale Train Box Car White Pass Yukon 27001
Bachmann On30 Scale Train Box Car Mineral Red Data Only 27098
Bachmann On30 Scale Train 18 Freight Cars Box Car 2 per box 26501
Bachmann On30 Scale Train Box Car Murphy Roof Oxide Red 2 box 26552
Bachmann On30 Scale Train Box Car South Pacific Coast Sunset Logo 27002
Bachmann On30 Scale Train 18 Box Car W Murphy Roof Gray 2 box 26553
HAWTHORNE VILLAGE On30 BACHMANN CONFEDERATE CIVIL WAR J E B STUART BOX CAR 005
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 315 nd
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 603
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 3145 n
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 606
BINKLEY KIT BUILT HOn3 NARROW GAUGE DENVER RIO GRANDE DRGW BOX CAR WAGON 15 nd
BINKLEY KIT BUILT HOn3 NARROW GAUGE DSPPRR DENVER SOUTH PARK REEFER BOX CAR 605
On30 On3 Pro built Wood Boxcar Model