The 2008 income statement and balance sheet (excerpts) for BNSF Railway are below.
BNSF Railway Company and Subsidiaries Consolidated Statements of Income
Year ended Dec 31, (In millions) 2008 2007 2006
Revenues $17,787 $15,610 $14,816
Operating expenses:
Fuel 4,640 3,327 2,856
Compensation and benefits 3,859 3,754 3,801
Purchased services 2,077 1,995 1,894
Depreciation and amortization 1,395 1,292 1,175
Equipment rents 901 942 930
Materials and other 1,019 790 618
Operating income 3,896 3,510 3,542
Interest expense 97 87 113
Interest income, related parties (19) (191) (162)
Other expense, net 18 31 39
Income before income taxes 3,800 3,583 3,552
Income tax expense 1,438 1,384 1,314
Net income $ 2,362 $ 2,199 $ 2,238

BNSF Railway Company and Subsidiaries Consolidated Balance Sheets (excerpts)
Dec 31, (In millions) 2008 2007
Cash and cash equivalents $ 209 $ 24
Accounts receivable, net of allowance of $54 and $34 873 805
Materials and supplies 524 579
Current portion of deferred income taxes 434 282
Other current assets 337 325
Total current assets 2,377 2,015

Property and equipment, net 30,838 29,560
Other assets 2,910 1,953
Total assets $36,125 $33,528

The footnotes included the following: Property & equipment, net (in millions) is as follows:
Dec 31, 2008 2007
Land $ 1,751 $ 1,718
Track structure 19,108 18,037
Other roadway 12,922 12,370
Locomotives 4,210 4,003
Freight cars and other equipment 2,140 2,034
Computer hardware and software 615 572
Total cost 40,746 38,734
Less accumulated depreciation and amortization (9,908) (9,174)
Property and equipment, net $30,838 $29,560
Required:
a. What proportion of total assets, does BNSF hold as property and equipment in 2008 and 2007?
b. What economic assets might not be reflected on the BNSF balance sheet?
c. By what percentage are the assets ‘used up' at the year-end 2008?