CP 2018 ANNUAL REPORT  /  86 The Company maintains an allowance for doubtful accounts based on expected collectability of accounts receivable. The Allowance for doubtful accounts is based on specific identification of uncollectable accounts, the application of historical percentages by aging category and an assessment of the current economic environment. 11 Dispositions of properties Gain on sale of Bass Lake railway line During the fourth quarter of 2018, the Company completed the sale of the Bass Lake railway line for gross proceeds of $37 million (U.S. $27 million). The company recorded a gain on sale of $35 million ($26 million after tax) within "Purchased services and other" from the transaction. During the fourth quarter of 2016, the Company completed the sale of its Obico rail yard for gross proceeds of $38 million. The Company recorded a gain on sale of $37 million ($33 million after tax) within "Purchased services and other" from the transaction. In March 2016, the Company completed the sale of CP’s Arbutus Corridor (the “Arbutus Corridor”) to the City of Vancouver for gross proceeds of $55 million. The agreement allows the Company to share in future proceeds on the eventual development and/or sale of certain parcels of the Arbutus Corridor. The Company recorded a gain on sale of $50 million ($43 million after tax) within "Purchased services and other" from the transaction during the first quarter of 2016. Gain on sale of Obico Gain on sale of Arbutus Corridor 12 Investments 13 Properties Capital leases included in properties (in millions of Canadian dollars) 2018 2017 Rail investments accounted for on an equity basis $ 160 $ 144 Other investments 43 38 Total investments $ 203 $ 182 (in millions of Canadian dollars except percentages) 2018 2018 2017 Weighted average annual depreciation rate Cost Accumulated depreciation Net book value Cost Accumulated depreciation Net book value Track and roadway 2.8% $ 18,599 $ 5,236 $ 13,363 $ 17,285 $ 4,814 $ 12,471 Buildings 3.0 % 781 218 563 719 196 523 Rolling stock 2.8% 4,467 1,613 2,854 4,114 1,557 2,557 Information systems (1) 10.4 % 551 252 299 551 264 287 Other 5.1% 1,984 645 1,339 1,760 582 1,178 Total $ 26,382 $ 7,964 $ 18,418 $ 24,429 $ 7,413 $ 17,016 $^{(1)}$ During 2018, CP capitalized costs attributable to the design and development of internal-use software in the amount of $53 million (2017 – $49 million; 2016 – $46 million). Current year depreciation expense related to internal use software was $49 million (2017 – $55 million; 2016 – $63 million). (in millions of Canadian dollars) 2018 2017 Cost Accumulated depreciation Net book value Cost Accumulated depreciation Net book value Buildings $ 1 $ 1 $ — $ 1 $ 1 $ — Rolling stock 311 124 187 311 115 196 Total assets held under capital lease $ 312 $ 125 $ 187 $ 312 $ 116 $ 196