Add rail_usage variable derived from rail_subsidy_spending#227
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Derives rail_usage (quantity at base year prices) from rail_subsidy_spending by dividing by the fare index for the survey year (2021): rail_usage = rail_subsidy_spending / 1.010 This enables policyengine-uk to properly decompose rail spending into price × quantity, allowing reforms to modify the fare_index parameter independently of usage quantity. Related: PolicyEngine/policyengine-uk#1406 Fixes: #226 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
- Import system from policyengine_uk to access parameters - Add get_fare_index_survey_year() function that tries to read gov.dft.rail.fare_index from parameters, falls back to 1.010 - This allows the code to work both before and after policyengine-uk PR #1406 is merged 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
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Summary
Adds a
rail_usagevariable to the dataset representing rail usage quantity at base year (2020) prices.Implementation
Where
RAIL_FARE_INDEX_SURVEY_YEAR = 1.010(the 2021 fare index from policyengine-uk).Why This Matters
This enables policyengine-uk PR #1406 to properly decompose rail spending into price × quantity:
Without this change, the policyengine-uk formula would be a tautology:
rail_usage = spending / fare_index(computed each year)rail_subsidy_spending = rail_usage × fare_index = spending(no change!)With this change,
rail_usageis derived once at the survey year and stored in the dataset. This allows reforms to modifyfare_indexand see actual effects onrail_subsidy_spending.Use Case
The Autumn Budget 2025 rail fares freeze policy freezes fares in 2026 while ridership continues to grow. Proper price × quantity decomposition enables:
fare_indexparameterrail_usageRelated
🤖 Generated with Claude Code